This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
Feedback from the HTML Accessibility Task force on WebVTT as per http://lists.w3.org/Archives/Public/public-tt/2015Apr/0049.html We have concerns regarding the impact on users of magnification with respect to several WebVTT specifications. Has the group considered its specification's impact on users of magnification? Items raising this concern included: * Snap to flag * Sizing of the captions rendering area * Visibility of captions when text is zoomed
The layout algorithm is designed to be robust against font size changes, if that's the kind of magnification we're talking about. It is possible to author captions to give them less horizontal space to grow than ideal so that you end up with too much line wrapping, so perhaps some authoring guidance on that topic would be in order.
Simple reply: yes, the group has considered the implications of magnification and has built the algorithms such that browsers will do the best they can to display caption text without overlap and in full. More details below. Given this, I'm going to suggest closing this bug. === Addressing the concerns here: * Snap to flag (assumption: snap-to-lines flag) -> Concern: the concern is likely that if text is explicitly positioned on certain lines, there is potential that enlarging the text may lead to text growing outside the rendering area (e.g. text positioned in the top-most line), or successive lines may grow into each other. -> Reply: First we have to understand that when content is enlarged, the video is typically enlarged together with the text. Therefore, the occurrance of this problem is rare. Secondly, there is a conflict between authoring requirements and rendering limitations. The browser rendering algorithm will as well as it can, but once text breaks out of the video rendering boundaries, there is not much it can do. You can check the consequences of this in the example at http://html5videoguide.net/BeginningHTML5Video/CH4/code_c4_17.html (it's build to automatically pause). * Sizing of the captions rendering area -> Concern: the concern is likely that the display area of captions is limited to the background area of the video element it is rendered onto and that with magnification the captions may go outside this rendering area. -> Reply: The area outside the video element is no usable to render captions onto (think about full-screen mode, or if the video is on a Web page there is other content around the video element). Therefore, after having done all it can to try and retain visibility of all caption text, the browser will hit the limit of what it can do. * Visibility of captions when text is zoomed -> Concern: the concern is likely about what happens when the text is zoomed, but the video isn't. ->Reply: If there are tools that do this, then you will hit the issues of overlapping text and disappearing text when hitting the boundaries of the rendering area faster than normal. However, it is unlikely that a tool would exist that zooms just the text and not the video element on screen. Normally, all content on a Web page is zoomed when magnification or zooming tools are in use.
MOVED TO GITHUB: https://github.com/w3c/webvtt/issues/426