This section provides examples of the accessibility
requirements of people with auditory, cognitive, neurological,
physical, speech, or visual disabilities to access and
comprehend media, including requirements for media formats and
media player technologies. For a broader exploration of how
people with different disabilities interact with web content and
tools, see How
People with Disabilities Use the Web.
1.1 Visual: Blindness
People who are blind cannot access visual information in
videos, player controls, status indicators, etc. They need
the information in an alternative representation of audio or
text. People who are blind use a screen reader and/or
refreshable Braille display, and media content needs to be
operable with these assistive technologies (ATs).
1.2 Visual: Low vision
People with low vision can use some visual information.
Depending on their visual
ability they might have specific issues such as difficulty discriminating
foreground information from background information, or discriminating colors.
Glare caused by excessive scattering in the eye can be a significant challenge,
especially for very bright content or surroundings. They may be unable
to react quickly to transient information, and may have a narrow angle
of view and so may not detect key information presented temporarily where
they are not looking, or in text that is moving or scrolling. A person
will likely use screen magnification software. This means that they will only be viewing
a portion of the screen, and so must manage tracking media content via
their AT. They may have difficulty reading when text is too small, has
poor background contrast (too high or too low), or when outlined or other fancy font types or
effects are used. If the font is an image, it is likely to appear grainy when magnified.
They may be using an AT that adjusts all the colors of
the screen, such as inverting the colors, so the media content must be
viewable through the AT. Users with low vision will often benefit from the same
text streams and instructions that are sometimes hidden or displayed off screen for
users of screen readers or refreshable Braille.
1.3 Visual: Atypical color perception
People with atypical color perception (often called
"color blindness") may not be able to
discriminate between different colors, or may miss
key information when coded with color only, such as colors in media
controls and text overlays.
1.4 Auditory: Deafness
People who are deaf generally cannot use audio. Thus, an alternative representation
is required, typically through synchronized captions and/or sign translation.
1.5 Auditory: Hard of hearing
People who are hard of hearing may be able to use some audio material,
but might not be able to discriminate certain types of sound, and may miss
any information presented as audio only if it contains frequencies they
can't hear, or is masked by background noise or distortion. They may miss
audio which is too quiet, or of poor quality. Speech may be challenging
if it is too fast and cannot be played back more slowly. Information presented
using multichannel audio (e.g., stereo) may not be perceived by people
who are deaf in one ear.
People with cochlear implants may not
have issues with audio volume levels, but comprehension may be
challenging if the media experience is overwhelming.
1.6 Auditory & Visual: Deaf-blind
Individuals who are deaf-blind have a combination of conditions that may
result in one of the following: blindness and deafness; blindness and difficulty
in hearing; low vision and deafness; or low vision and difficulty in hearing.
Depending on their combination of conditions, individuals who are deaf-blind
may need captions that can be enlarged, changed to high-contrast colors,
or otherwise styled; or they may need captions and/or described video that
can be presented with AT (e.g., a refreshable Braille display). They may
need synchronized captions and/or described video, or they may need a non-time-based
transcript which they can read at their own pace.
1.7 Physical impairment
Some people with physical disabilities such as limited
muscle control (including tremors, lack of coordination, and
paralysis), pain that impedes movement, or missing limbs
cannot use a keyboard or mouse to interact with content and
controls. Some use a keyboard but not a pointing device,
some use a switch with an on-screen keyboard, and some use
other assistive technology. The media player must be usable
with only a keyboard, including access to all player
controls and methods for selecting alternative content.
1.8 Cognitive disabilities
Cognitive disabilities include a wide range of conditions
that may include intellectual disabilities (called learning disabilities
in some regions), autism-spectrum disorders, memory impairments, mental-health
disabilities, attention-deficit disorders, audio- and/or visual-perceptive
disorders, dyslexia and dyscalculia (called learning disabilities in some
regions), or seizure disorders. The accessibility supports for these different
conditions vary widely.
Individuals with some conditions may process
information aurally better than by reading text; therefore, information
that is presented as text embedded in a video should also be available
as audio descriptions. Individuals with other conditions may need to reduce
distractions or flashing in presentations of video. Some conditions, such
as autism-spectrum disorders, may have multisystem effects. Individuals
may need a combination of different accommodations.
Overall, the media experience for people on the autism spectrum should
be customizable and well designed so as to not be overwhelming. Care
must be taken to present a media experience that focuses on the purpose
of the content and provides alternative content in a clear, concise manner.
2. Alternative Content Technologies
A number of alternative content types have been developed to help users
with sensory disabilities gain access to audio-visual content. This section
lists them, explains generally what they are, and provides a number of requirements
on each that need to be satisfied with technology developed in HTML5 around
the media elements.
2.1 Described video
Described video contains descriptive narration of key visual elements
designed to make visual media accessible to people who are blind or visually
impaired. The descriptions include actions, costumes, gestures, scene changes
or any other important visual information that someone who cannot see the
screen might ordinarily miss. Descriptions are traditionally audio recordings
timed and recorded to fit into natural pauses in the program, although
they may also briefly obscure the main audio track (see the section on
extended descriptions for an alternative approach).
As with captions, descriptions can be open or closed.
Open descriptions are merged with the program-audio
track and cannot be turned off by the viewer.
Closed descriptions can be turned on and off by the
viewer. They can be recorded as a separate track containing descriptions
only, timed to play at specific spots in the timeline and played in parallel
with the program-audio track.
- Some descriptions can be delivered as a separate audio channel
mixed in at the player.
- Other options include a computer-generated ‘text to speech’
track, also known as text video descriptions. This is described
in the next subsection.
Described video provides benefits that reach beyond blind or visually
impaired viewers; e.g., students grappling with difficult materials or
concepts. Descriptions can be used to give supplemental information about
what is on screen—the structure of lengthy mathematical equations or the
intricacies of a painting, for example.
Described video is available on some television programs and in many movie
theaters in the U.S. and other countries. Regulations in the U.S. and Europe
are increasingly focusing on description, especially for television, reflecting
its priority with citizens who have visual impairments. The technology
needed to deliver and render basic video descriptions is in fact relatively
straightforward, being an extension of common audio-processing solutions.
Playback products must support multi-audio channels required for description,
and any product dealing with broadcast TV content must provide adequate
support for descriptions. Descriptions can also provide text that can be
indexed and searched.
Systems supporting described video where the descriptions are available as
independent file or channel resources must:
[DV-1] Provide an indication that descriptions are available, and
[DV-2] Render descriptions in a time-synchronized manner, using
the primary media resource as the timebase master.
[DV-3] Support multiple description tracks (e.g., discrete tracks
containing different levels of detail).
[DV-4] Support recordings of high quality speech as a track of the media resource, or as an external file.
[DV-5] Allow the author to independently adjust the volumes of the
audio description and original soundtracks where these are available as separate audio channel resources.
[DV-6] Allow the user to independently adjust the volumes of the
audio description and original soundtracks (where these are available as separate audio channel resources),
with the user's settings overriding the author's.
[DV-7] Permit smooth changes in volume rather than stepped changes.
The degree and speed of volume change should be under user control.
[DV-8] Allow the author to provide fade and pan controls to be accurately
synchronized with the original soundtrack.
[DV-9] Allow the author to use a codec which is optimized for voice
only, rather than requiring the same codec as the original soundtrack.
[DV-10] Allow the user to select from among different languages
of descriptions, if available, even if they are different from the
language of the main soundtrack.
[DV-11]Support the simultaneous playback of
both the video description track and primary audio resource tracks so that either may be
directed at separate outputs (e.g., one to loudspeakers and the other to headphones). Where a screen reader is present,
also support the ability to keep its audio output separate from both the described video track and the primary audio
the user to relocate the pan location of the various audio tracks within
the audio field, with the user setting overriding the author setting.
The setting should be re-adjustable as the media plays.
[DV-13] Support metadata, such as copyright information, usage rights,
2.2 Text video description
Described video that uses text for the description source rather than
a recorded voice creates specific requirements.
Text video descriptions (TVDs) are delivered to the client as text and
rendered locally by assistive technology such as a screen reader or a Braille
device. This can have advantages for screen-reader users who want full
control of the preferred voice and speaking rate, or other options to control
the speech synthesis.
Text video descriptions are provided as text files containing start times
for each description cue. Since the duration that a screen reader takes
to read out a description cannot be determined during authoring of the
cues, it is difficult to ensure they don't obscure the main audio or other
description cues. This is likely to be caused by at least three reasons:
- An author of text video descriptions does not have a screen reader.
This means s/he cannot check if the description fits within the time
frame. Even if s/he has a screen reader, a user's screen reader will
be set to a different reading speed and may take longer to read the same
- Some screen-reader users (e.g., those who are elderly or have learning
disabilities) may slow down the speech rate.
- A visually complicated scene (e.g., figures on a blackboard in an
online physics class) may require more description time than is available
in the program-audio track.
People with low-vision may also benefit from having access to text video descriptions.
Systems supporting text video descriptions must:
[TVD-1] Support presentation of text video descriptions through
a screen reader, Braille device and/or modified print with playback speed control,
voice control and synchronization points within the video.
[TVD-2] TVDs need to be provided in a format that contains the following
- start time, text per description cue (the duration is determined
dynamically, though an end time could provide a cut point)
- possibly a speech-synthesis markup to improve quality of
the description (existing speech synthesis markups include SSML and CSS 3 Speech Module)
- accompanying metadata providing labeling for speakers, language,
- visual style markup (see section on Captioning).
[TVD-3] Where possible, provide a text or separate audio track privately
to those that need it in a mixed-viewing situation, e.g., through headphones.
[TVD-4] Where possible, provide options for authors and users to
deal with the overflow case: continue reading, stop reading, and pause
the video. (One solution from a user's point of view may be to pause
the video and finish reading the TVD, for example.) User preference
should override authored option.
[TVD-5] Support the control over speech-synthesis playback speed,
volume and voice, and provide synchronization points with the video.
2.3 Extended video descriptions
Video descriptions are usually provided as recorded speech, timed to play
in the natural pauses in dialog or narration. In some types of material,
however, there is not enough time to present sufficient descriptions. To
meet such cases, the concept of extended description was developed. Extended
descriptions work by pausing the video and program audio at key moments,
playing a longer description than would normally be permitted, and then
resuming playback when the description is finished playing. This will naturally
extend the timeline of the entire presentation. This procedure has not
been possible in broadcast television; however, hard-disk recording and
on-demand Internet systems can make this a practical possibility.
Extended video description (EVD) has been reported to have benefits for
cognitive disabilities; for example, it might benefit people
Syndrome and other Autistic Spectrum Disorders, in that it can make connections
between cause and effect, point out what is important to look at, or explain
moods that might otherwise be missed.
Systems supporting extended audio descriptions must:
Support detailed user control as specified in [TVD-4]
extended video descriptions.
[EVD-2] Support automatically pausing the video and main audio tracks
in order to play a lengthy description.
[EVD-3] Support resuming playback of video and main audio tracks
when the description is finished.
Because the user is the ultimate arbiter of the rate at which TTS
playback occurs, it is not feasible for an author to guarantee that any
texted audio description can be played within the natural pauses in
dialog or narration of the primary audio resource. Therefore, all texted
descriptions must be treated as extended text descriptions potentially
requiring the pausing and resumption of primary resource playback.
2.4 Clean audio
A relatively recent development in television accessibility is the concept
audio, which takes advantage of the increased adoption of multichannel
audio. This is primarily aimed at audiences who are hard of hearing, and
consists of isolating the audio channel containing the spoken dialog and
important non-speech information that can then be amplified or otherwise
modified, while other channels containing music or ambient sounds are attenuated.
Using the isolated audio track may make it possible to apply more sophisticated
audio processing such as pre-emphasis filters, pitch-shifting, and so on
to tailor the audio to the user's needs, since hearing loss is typically
frequency-dependent, and the user may have usable hearing in some bands
yet none at all in others.
Systems supporting clean audio and multiple audio tracks must:
[CA-1] Support clean
audio as a separate, alternative audio track from other audio-based
alternative media resources, including the primary audio resource.
[CA-2] Support the synchronization of multitrack audio either within
the same file or from separate files - preferably both.
[CA-3] Support separate volume control of the different audio tracks.
[CA-4] Support pre-emphasis filters, pitch-shifting, and other audio-processing
For people who are deaf or hard-of-hearing, captioning is a prime alternative
representation of audio. Captions are in the same language as the main
audio track and, in contrast to foreign-language subtitles, render a transcription
of dialog or narration as well as important non-speech information, such
as sound effects, music, and laughter. Historically, captions have been
either closed or open. Closed captions have been transmitted as data along
with the video but were not visible until the user elected to turn them
on, usually by invoking an on-screen control or menu selection. Open captions
have always been visible; they had been merged with the video track and
could not be turned off.
Ideally, captions should be a verbatim representation of the audio; however,
captions are sometimes edited for various reasons— for example, for reading
speed or for language level. In general, consumers of captions have expressed
that the text should represent exactly what is in the audio track. If edited
captions are provided, then they should be clearly marked as such, and
the full verbatim version should also be available as an option.
The timing of caption text can coincide with the mouth movement of the
speaker (where visible), but this is not strictly necessary. For timing
purposes, captions may sometimes precede or extend slightly after the audio
they represent. Captioning should also use adequate means to distinguish
between speakers as turn-taking occurs during conversation; this has in
the past been done by positioning the text near the speaker, by associating
different colors to different speakers, or by putting the name and a colon
in front of the text line of a speaker.
Captions are useful to a wide array of users in addition to their originally
intended audiences. Gyms, bars, and restaurants regularly employ captions
as a way for patrons to watch television while in those establishments.
People learning to read or learning the language of the country where they
live as a second language also benefit from captions: research has shown
that captions help reinforce vocabulary and language. Captions can also
provide a powerful search capability, allowing users and search engines
to search the caption text to locate a specific video or an exact point
in a video.
Formats for captions, subtitles or foreign-language subtitles must:
[CC-1] Render text in a time-synchronized manner,
using the media resource as the timebase master.
Most of the time, the main audio track would be the best candidate
for the timebase. Where a video without audio, but with a text track,
is available, the video track becomes the timebase master. Also, there
may be situations where an explicit timing track is available.
[CC-2] Allow the author to specify erasures, i.e.,
times when no text is displayed on the screen (no text cues are active).
This should be possible both within media resources and caption
[CC-3] Allow the author to assign timestamps so
that one caption/subtitle follows another, with no perceivable gap
This means that caption cues should be able to either let the
start time of the subsequent cue be determined by the duration of the
cue or have the end time be implied by the start of the next cue. For
overlapping captions, explicit start and end times are then required.
[CC-4] Be available in a text encoding.
This means that determined character encodings should be supported
- which could be either by making the character encoding explicit or
by enforcing a single default one such as UTF-8.
[CC-5] Support positioning in all parts of the
screen - either inside the media viewport but also possibly in a determined
space next to the media viewport. This is particularly important when
multiple captions are on screen at the same time and relate to different
speakers, or when in-picture text is avoided.
The minimum requirement is a bounding box (with an optional background)
into which text is flowed, and that probably needs to be pixel aligned.
The absolute position of text within the bounding box is less critical,
although it is important to be able to avoid bad word-breaks and have
adequate white space around letters and so on. There is more on this
in a separate requirement.
The caption format could provide a min-width/min-height for its bounding
box, which typically is calculated from the bottom of the video viewport,
but can be placed elsewhere by the web page, with the web page being
able to make that box larger and scale the text relatively, too. The
positions inside the box should probably be into regions, such as top,
right, bottom, left, center.
[CC-6] Support the display of multiple regions
of text simultaneously.
This typically relates to multiple text cues that are defined
on overlapping times. If the cues' rendering target are made out to different
spatial regions, they can be displayed simultaneously.
[CC-7] Display multiple rows of text when rendered
as text in a right-to-left or left-to-right language.
Internationalization is important not just for subtitles, as captions
can be used in all languages.
[CC-8] Allow the author to specify line breaks.
[CC-9] Permit a range of font faces and sizes.
The legibility of the rendered text depends upon the
size of the text as perceived by the viewer. This is in turn dependent upon the display
size and the distance between the display and the viewer. Users must be able to select an
appropriate format for their environment. See also CC-11 below.
[CC-10] Render a background in a range of colors,
supporting a full range of opacity levels.
text in a range of colors. The user should have final control over rendering styles like
color and fonts; e.g., through user preferences.
A default palette of colors suitable for users with atypical
should be available to distinguish editorial concepts such as speakers.
There are likely to be conflicting requirements between different users
with differing cognitive conditions to maximize the accessibility of
content, so full color customization should be available. For example
users with cognitive conditions such as dyslexia (itself an umbrella label
for a variety of conditions), ADHD,
and Asperger's may find that viewing
content that is given a particular color cast, akin to viewing through
blue eyeglasses, helps them to read presented text.
While users must have the ability to customize their experience,
it is preferable that developers do
their best to ensure the legibility of the content they are presenting. For example, the
use of Media Queries and alternate style sheets based upon screen size
is a common technique for tuning the
size and style of fonts used depending on the output device (e.g., a large monitor vs. a
small smart-phone screen). Ideally, a combination of techniques such as this along with
sensible system-provided defaults will reduce the need for end-users to customize beyond
general system settings.
[CC-12] Enable rendering of text with a thicker
outline or a drop shadow to allow for better contrast with the background.
The use of drop shadows is not a suitable general alternative to
displaying text on a non-transparent background. For example, white text with drop
shadows on a transparent background is not legible over white content (e.g., footage of
The use of drop shadows can increase the sense of 'busyness', and can have
negative impacts upon viewers with some cognitive conditions. In general developers will
improve text legibility of they avoid the use of drop shadows.
[CC-13] Where a
background is used, it should be possible
to keep the caption background visible even in times where no text
is displayed, such that it minimizes distraction. However, where captions
are infrequent the background should be allowed to disappear to enable
the user to see as much of the underlying video as possible.
It may be technically possible to have cues without text.
[CC-14] Allow the use of mixed display styles—
e.g., mixing paint-on captions with pop-on captions— within a single
caption cue or in the caption stream as a whole. Pop-on captions are
usually one or two lines of captions that appear on screen and remain
visible for one to several seconds before they disappear. Paint-on
captions are individual characters that are "painted on" from
left to right, not popped onto the screen all at once, and usually
are verbatim. Another often-used caption style in live captioning is
roll-up - here, cue text follows double chevrons ("greater than" symbols),
and is used to identify different speakers. Each sentence "rolls
up" to about three lines. The top line of the three disappears
as a new bottom line is added, allowing the continuous rolling up of
new lines of captions.
When displaying captions using the paint-on style, it is important to ensure
that the final words that are displayed are visible for enough time for them to be read.
Similarly, in karaoke, individual characters are often "painted
The comprehension and appreciation of captions and subtitles depends on
how well matched they are 'editorially' to the related video content. In
particular the pacing of the content should be reflected in the caption
text; for example a fast paced drama is likely to benefit from
relatively short captions that change more often in comparison to a slow
paced one. In the most extreme case, very fast changing short subtitles can cause
readability problems because they can prevent viewers from having enough
attention to consider the video; such extremes should be avoided.
positioning such that the edge of the captions is a sufficient distance from the
nearest screen edge to permit readability (e.g., at least 1/12 of the total screen height above
the bottom of the screen, when rendered as text in a right-to-left or left-to-right
[CC-16] Use conventions that include inserting
left-to-right and right-to-left segments within a vertical run (e.g.
Tate-chu-yoko in Japanese), when rendered as text in a top-to-bottom
[CC-17] Represent content of different natural
languages. In some cases the inclusion of a few foreign words forms
part of the original soundtrack, and thus needs to be so indicated in the caption.
Also allow for separate caption files for different languages
and on-the-fly switching between them. This is also a requirement for
subtitles. See also [CC-20]
Caption/subtitle files that are alternatives in different languages
are probably better provided in different caption resources and should be user
selectable. Realistically, no more than 2 languages should be present at the same time on the screen.
[CC-18] Represent content of at least those specific
natural languages that may be represented with [Unicode 3.2], including
common typographical conventions of that language (e.g., through the
use of furigana and other forms of ruby text).
[CC-19] Present the full range of typographical
glyphs, layout and punctuation marks normally associated with the natural
language's print-writing system.
[CC-20] Permit in-line mark-up for foreign words
Italics markup may be sufficient for a human user, but it is important
to be able to mark up languages so that the text can be rendered correctly,
since the same Unicode can be shared between languages and rendered differently
in different contexts. This is mainly an localization issue. It is also important
for audio rendering, to get correct pronunciation.
[CC-21] Permit the distinction between different
Further, systems that support captions must:
[CC-22] Support captions that are provided inside
media resources as tracks, or in external files.
It is desirable to expose the same API to both.
[CC-23] Ascertain that captions are displayed in
sync with the media resource.
[CC-24] Support user activation/deactivation of
This requires a menu of some sort that displays the available
tracks for activation/deactivation.
[CC-25] Support both edited and verbatim
captions when available.
Edited and verbatim captions may be provided in two separate
caption resources. How these differ should be explained to the user.
[CC-26] Support multiple tracks of
foreign-language subtitles including multiple subtitle tracks in the same foreign language.
These different-language "tracks" can be provided in
[CC-27] Support live-captioning functionality.
[CC-28] Enable the
bounding box of the background area to be extended by a preset distance relative to the
foreground text contained with that background area.
2.7 Enhanced captions/subtitles
Enhanced captions are timed text cues that have been enriched with further
information - examples are glossary definitions for acronyms and other
initialisms, foreign terms (for example, Latin), jargon or descriptions
for other difficult language. They may be age-graded, so that multiple
caption tracks are supplied, or the glossary function may be added dynamically
through machine lookup.
Glossary information can be added in the normal time allotted for the
cue (e.g., as a callout or other overlay), or it might take the form of
a hyperlink that, when activated, pauses the main content and allows access
to more complete explanatory material.
Such extensions can provide important additional information to the content
that will enable or improve the understanding of the main content to users of assistive
technology. Enhanced text cues will be particularly useful for those with restricted
reading skills, to subtitle users, and to caption users. Users may often
come across keywords in text cues that lend themselves to further in-depth
information or hyperlinks, such as an e-mail contact or phone number for
a person, a unfamiliar term that needs a link to a definition, or
an idiom that needs comments to explain it to a foreign-language speaker.
Systems that support enhanced captions must:
[ECC-1] Support metadata markup for (sections of)
timed text cues.
Such "metadata" markup can be realized through a title
attribute on a <span> of the text, or a hyperlink to another location
where a term is explained, an <abbr> element, an <acronym> element,
a <dfn> element, or through RDFa or microdata.
[ECC-2] Support hyperlinks and other activation
mechanisms for supplementary data for (sections of) caption text.
This could be realized by including hyperlinks or
buttons into timed text cues, where additional overlays could be created
or a different page loaded. One needs to deal here with the need to
pause the media timeline for reading of the additional information.
[ECC-3]Support text cues that may be longer
than the time available until the
next text cue, thus providing overlapping text cues. In such instances,
users should be enabled to decide whether they prefer to see overlapping
text, or automatically shorten display time, or to have the media
resource paused while the caption is displayed. Timing should be
provided by the author, but the user should always be able to override
the author's timings.
This feature is analogous to extended video descriptions - where timing
for a text cue is longer than the available time for the cue. It may be
necessary to halt the media to allow the user more time to read the text
and its additional material. In such cases the pause is dependent on the
user's reading speed, so this implies user control or automated
[ECC-4] Support timed text cues that are
allowed to overlap with each other in
time and be present on screen at the same time (e.g., those that come
from the speech of different individuals). Also support timed text cues that are
not allowed to overlap, so that playback will be paused in order to
allow users to catch up with their reading.
[ECC-5] Allow users to define the reading speed
and thus define how long each text cue requires, and whether media
playback needs to pause sometimes to let them catch up on their reading.
This can be a setting in the UA, which will define user-interface
2.8 Sign translation
Sign language shares the same concept as captioning: it presents both
speech and non-speech information in an alternative format. Note that due
to the wide regional variation in signing systems (e.g., American Sign
Language vs British Sign Language), sign translation may not be appropriate
for content with a global audience unless localized variants can be made
Signing can be open, mixed with the video and offered as an entirely alternative
stream or closed (using some form of picture-in-picture or alpha-blending
technology). It is possible to use quite low bit rates for much of the
signing track, but it is important that facial, arm, hand and other body
gestures be delivered at sufficient resolution to support legibility. Animated
avatars may not currently be sufficient as a substitute for human signers,
although research continues in this area and it may become practical at
some point in the future.
Acknowledging that not all devices will be capable of handling multiple
video streams, this is a SHOULD requirement for browsers where hardware
is capable of support. Strong authoring guidance for content creators will
mitigate situations where user-agents are unable to support multiple video
streams (WCAG) - for example, on mobile devices that cannot support multiple
streams, authors should be encouraged to offer two versions of the media
stream, including one with signed captions burned into the media.
Selecting from multiple tracks for different sign languages should be
achieved in the same fashion that multiple caption/subtitle files are handled.
Systems supporting sign language must:
[SL-1] Support sign-language video either as a track as part of
a media resource or as an external file.
[SL-2] Support the synchronized playback of the sign-language video
with the media resource.
[SL-3] Support the display of sign-language video either as picture-in-picture
or alpha-blended overlay, as parallel video, or as the main video with
the original video as picture-in-picture or alpha-blended overlay.
Parallel video here means two discrete videos playing in sync with
each other. It is preferable to have one discrete <video> element
contain all pieces for sync purposes rather than specifying multiple <video> elements
intended to work in sync.
[SL-4] Support multiple sign-language tracks in several sign languages.
[SL-5] Support the interactive activation/deactivation of a sign-language
track by the user.
While synchronized captions are generally preferable for people with hearing
impairments, for some users they are not viable – those who are deaf-blind,
for example, or those with cognitive or reading impairments that make it
impossible to follow synchronized captions. And even with ordinary captions,
it is possible to miss some information as the captions and the video require
two separate loci of attention. The full transcript supports different
user needs and is not a replacement for captioning. A transcript can either
be presented simultaneously with the media material, which can assist slower
readers or those who need more time to reference context, but it should
also be made available independently of the media.
A full text transcript should include information that would be in both
the caption and video description, so that it is a complete representation
of the material, as well as containing any interactive options.
Systems supporting transcripts must:
Support the provisioning of a full text transcript for the
media asset in a separate but linked resource, where the linkage is
programmatically accessible to AT
Support the provisioning of both scrolling and static display
of a full text transcript with the media resource, e.g., in an area next
to the video or underneath the video, which is also AT
[T-3] Allow the user to customize the visual rendering of the full
text transcript, e.g., font, font size, foreground and background color, line, letter, and word spacing.
3. System Requirements
3.3 Time-scale modification
While all devices may not support the capability, a standard control API
must support the ability to speed up or slow down content presentation
without altering audio pitch.
While perhaps unfamiliar to some, this feature has been present
on many devices, especially audiobook players, for some 20 years now.
The user can adjust the playback rate of prerecorded time-based media
content, such that all of the following are true
[TSM-1] The user can adjust the playback rate of the time-based
media tracks to between 50% and 250% of real time.
[TSM-2] Speech whose playback rate has been adjusted by the user
maintains pitch in order to limit degradation of the speech quality.
[TSM-3] All provided alternative media tracks remain synchronized
across this required range of playback rates.
[TSM-4] The user agent provides a function that resets the playback
rate to normal (100%).
[TSM-5] The user can stop, pause, and resume rendered audio and
animation content (including video and animated images) that last three
or more seconds at their default playback rate.
3.4 Production practice and resulting requirements
One of the biggest challenges to date has been the lack of a universal system
for media access. In response to user requirements various countries and
groups have defined systems to provide accessibility, especially captioning
for television. However these systems are typically not compatible. In
some cases the formats can be inter-converted, but some formats — for example
DVD sub-pictures — are image based and are difficult to convert to text.
Caption formats are often geared towards delivery of the media, for example
as part of a television broadcast. They are not well suited to the production
phases of media creation. Media creators have developed their own internal
formats which are more amenable to the editing phase, but to date there
has been no common format that allows interchange of this data.
Any media based solution should attempt to reduce as far as possible layers
of translation between production and delivery.
In general captioners use a proprietary workstation to prepare caption
files; these can often export to various standard broadcast ingest formats,
but in general files are not inter-convertible. Most video editing suites
are not set up to preserve captioning, and so this has typically to be
added after the final edit is decided on; furthermore since this work is
often outsourced, the copyright holder may not hold the final editable
version of the captions. Thus when programming is later re-purposed, e.g.
a shorter edit is made, or a ‘directors cut’ produced, the captioning may
have to be redone in its entirety. Similarly, and particularly for news
footage, parts of the media may go to web before the final TV edit is made,
and thus the captions that are produced for the final TV edit are not available
for the web version.
It is important when purchasing or commissioning media, that captioning
and described video is taken into account and made equal priority in terms
of ownership, rights of use, etc., as the video and audio itself.
This is primarily an authoring requirement. It is understood that a
common time-stamp format must be declared in HTML5, so that authoring tools
can conform to a required output.
Systems supporting accessibility needs for media must:
[PP-1] Support existing production practice for alternative content
resources, in particular allow for the association of separate alternative
content resources to media resources. Browsers cannot support all forms
of time-stamp formats out there, just as they cannot support all forms
of image formats (etc.). This necessitates a clear and unambiguous
declared format, so that existing authoring tools can be configured
to export finished files in the required format.
[PP-2] Support the association of authoring and rights metadata
with alternative content resources, including copyright and usage information.
[PP-3] Support the simple replacement of alternative content resources
even after publishing. This is again dependent on authoring practice
- if the content creator delivers a final media file that contains
related accessibility content inside the media wrapper (for example
an MP4 file), then it will require an appropriate third-party authoring
tool to make changes to that file - it cannot be demanded of the browser
to do so.
[PP-4] Typically, alternative content resources are created by different
entities to the ones that create the media content. They may even be
in different countries and not be allowed to re-publish the other one's
content. It is important to be able to host these resources separately,
associate them together through the web page author, and eventually
play them back synchronously to the user.
3.5 Discovery and activation/deactivation of available alternative content
by the user
As described above, individuals need a variety of media (alternative content)
in order to perceive and understand the content. The author or some web
mechanism provides the alternative content. This alternative content may
be part of the original content, embedded within the media container as
'fallback content', or linked from the original content. The user is faced
with discovering the availability of alternative content.
Alternative content must be both discoverable by the user, and accessible
in device agnostic ways. The development of APIs and user-agent controls
should adhere to the following UAAG guidance:
The user agent can facilitate the discovery of alternative content by
following these criteria:
[DAC-1] The user has the ability to have indicators rendered along
with rendered elements that have alternative content (e.g., visual
icons rendered in proximity of content which has short text alternatives,
long descriptions, or captions). In cases where the alternative content
has different dimensions than the original content, the user has the
option to specify how the layout/reflow of the document should be handled.
[DAC-2] The user has a global option to specify which types of alternative
content by default and, in cases where the alternative content has
different dimensions than the original content, how the layout/reflow
of the document should be handled.
[DAC-3] The user can browse the alternatives and switch between
[DAC-4] Synchronized alternatives for time-based media (e.g., captions,
descriptions, sign language) can be rendered at the same time as their
associated audio tracks and visual tracks
[DAC-5] Non-synchronized alternatives (e.g., short text alternatives,
long descriptions) can be rendered as replacements for the original
[DAC-6] Provide the user with the global option to configure a cascade
of types of alternatives to render by default, in case a preferred
alternative content type is unavailable
[DAC-7] During time-based media playback, the user can determine
which tracks are available and select or deselect tracks. These selections
may override global default settings for captions, descriptions, etc.
[DAC-8] Provide the user with the option to load time-based media
content such that the first frame is displayed (if video), but the
content is not played until explicit user request.
[DAC-9] Provide the user with the option to record alternative content
along with the primary content on devices where recording is available.
This feature can be user configurable to allow maximum flexibility in trading off the
anticipated future need for the description against the amount of extra data storage required. A flexible
solution giving maximum control to the user would be to provide a global setting with the following
- Always record the alternative content (the best default option, since a resource recorded by one user may later be
accessed by another different user who may have different and unanticipated requirements);
- Record the alternative content only if it is active at the time of recording;
- Ask at recording time whether to record the alternative content;
- Never record the alternative content.
3.6 Requirements on making properties available to the accessibility interface
Often forgotten in media systems, especially with the newer forms of packaging
such as DVD menus and on-screen program guides, is the fact that the user
needs to actually get to the content, control its playback, and turn on
any required accessibility options. For user agents supporting accessibility
APIs implemented for a platform, any media controls need to be connected
to that API.
On self-contained products that do not support assistive technology, any
menus in the content need to provide information in alternative formats
(e.g., talking menus). Products with a separate remote control, or that
are self-contained boxes, should ensure the physical design does not block
access, and should make accessibility controls, such as the closed-caption
toggle, as prominent as the volume or channel controls.
[API-1] The existence of alternative-content tracks for a media
resource must be exposed to the user agent.
[API-2] Since authors will need access to the alternative content
tracks, the structure needs to be exposed to authors as well, which
requires a dynamic interface.
[API-3] Accessibility APIs need to gain access to alternative content
tracks no matter whether those content tracks come from within a resource
or are combined through markup on the page.
3.7 Requirements on the use of the viewport
The video viewport plays a particularly important role with respect to
alternative-content technologies. Mostly it provides a bounding box for
many of the visually represented alternative-content technologies (e.g.,
captions, hierarchical navigation points, sign language), although some
alternative content does not rely on a viewport (e.g., full transcripts,
One key principle to remember when designing player ‘skins’ is that the
lower-third of the video may be needed for caption text. Caption consumers
rely on being able to make fast eye movements between the captions and
the video content. If the captions are in a non-standard place, this may
cause viewers to miss information. The use of this area for things such
as transport controls, while appealing aesthetically, may lead to accessibility
[VP-1] It must be possible to deal with three different
cases for the relation between the viewport size, the position of media
and of alternative content:
- the alternative content's extent is specified in relation
to the media viewport (e.g., picture-in-picture video, lower-third
- the alternative content has its own independent extent,
but is positioned in relation to the media viewport (e.g., captions
above the audio, sign-language video above the audio, navigation
points below the controls)
- the alternative content has its own independent extent and
doesn't need to be rendered in any relation to the media viewport
(e.g., text transcripts)
If alternative content has a different height or width than the media
content, then the user agent will reflow the (HTML) viewport.
This may create a need to provide an author hint to the web page
when embedding alternative content in order to instruct the web page how
to render the content: to scale with the media resource, scale independently,
or provide a position hint in relation to the media. On small devices
where the video takes up the full viewport, only limited rendering choices
may be possible, such that the UA may need to override author preferences.
[VP-2] The user can change the following characteristics
of visually rendered text content, overriding those specified by the
author or user-agent defaults (Note: this should
include captions and any text rendered in relation to media elements,
so as to be able to magnify and simplify rendered text):
- text scale (i.e., the general size of text),
- font family
- text color (i.e., foreground and background)
- letter spacing (tracking and kerning)
- line spacing (or line height), and
- word spacing.
[VP-3] Provide the user with the ability to adjust
the size of the time-based media up to the full height or width of
the containing viewport, with the ability to preserve aspect ratio
and to adjust the size of the playback viewport to avoid cropping,
within the scaling limitations imposed by the media itself.
This can be achieved by simply zooming into the web page, which
will automatically rescale the layout and reflow the content.
[VP-4] Provide the user with the ability to control
the contrast and brightness of the content within the playback viewport.
This is a user-agent device requirement and should already be
addressed in the UAAG. In live content, it may even be possible to adjust
camera settings to achieve this requirement. It is also a "SHOULD" level
requirement, since it does not account for limitations of various devices.
[VP-5] Captions and subtitles traditionally occupy
the lower third of the video, where controls are also usually
rendered. The user agent must avoid overlapping of overlay content
and controls on media resources. This must also happen if, for example,
the controls are only visible on demand.
If there are several types of overlapping overlays (including
captions and subtitles), implementations should attempt to ensure that none of
them overlaps with editorially important content.
particular, user agents
should avoid obscuring video components such as mouths, "burned in text" (embedded
captions or other annotations in the main video stream), etc. When in
constrained environments where it is impossible to avoid obscuring all of these
components, user agents should make every effort to avoid the most important of
them. Users typically expect
controls to appear at the bottom of the viewport. Controls should not be prevented from
becoming usable due to repositioning.
3.8 Requirements on secondary screens and other devices
Multiple secondary user devices must be directly addressable. This
functionality is increasingly also known by the new term, "Second Screen,"
even though there may be more than two screens in any given viewing
environment, and even though not all secondary devices are video displays. It
must be assumed that many users will have at least one additional display
device (such as a tablet), and/or at least one additional audio output device
(such as a Bluetooth headset) attached to a primary video display device, an
individual computer, or locally addressable on a LAN. It
must be possible to configure certain types of media for presentation on
specific devices, and these configuration settings must be readily overwritable
on a case-by-case basis by users.
Systems supporting secondary devices must:
[SD-1] Support a platform-accessibility architecture relevant to
the operating environment.
[SD-2] Ensure accessibility of all user-interface components including
the user interface, rendered content, and alternative content; make
available the name, role, state, value, and description via a platform-accessibility
[SD-3] If a feature is not supported by the accessibility architecture(s),
provide an equivalent feature that does support the accessibility architecture(s).
Document the equivalent feature in the conformance claim.
[SD-4] If the user agent implements one or more DOMs, they must
be made programmatically available to assistive technologies. This assumes the video element will write to the DOM.
[SD-5] If the user can modify the state or value of a piece of content
through the user interface (e.g., by checking a box or editing a text
area), the same degree of write access is available programmatically
[SD-6] If any of the following properties are supported by the accessibility-platform
architecture, make the properties available to the accessibility-platform
- the bounding dimensions and coordinates of rendered graphical
- font family;
- font size;
- text foreground color;
- text background color;
- change state/value notifications.
[SD-7] Ensure that programmatic exchanges between APIs proceed at
a rate such that users do not perceive a delay.