HTML5 includes new input types and attributes, such as type="tel", pattern, and placeholder. These new types and attribute have usability and accessibility
implications, and guidance within the HTML5 specification is at times contradictory. 

The pattern attribute
From the spec
The pattern attribute specifies a regular expression against which the control's value, or, when the multiple attribute applies and is set, the control's
values, are to be checked.
http://dev.w3.org/html5/spec/Overview.html#the-pattern-attribute

Comment:
The pattern attribute value, in regular expression notation, is not suitable as a hint or description for end-users. The HTML specification describes methods
for presenting the pattern to end-users: 

From the spec:
When an input element has a pattern attribute specified, authors should include a title attribute to give a description of the pattern. User agents may
use the contents of this attribute, if it is present, when informing the user that the pattern is not matched, or at any other suitable time, such as in
a tooltip or read out by assistive technology when the control gains focus.


The specification goes on to state: 


When a control has a pattern attribute, the title attribute, if used, must describe the pattern. Additional information could also be included, so long
as it assists the user in filling in the control. Otherwise, assistive technology would be impaired. For instance, if the title attribute contained the
caption of the control, assistive technology could end up saying something like The text you have entered does not match the required pattern. Birthday,
which is not useful. UAs may still show the title in non-error situations (for example, as a tooltip when hovering over the control), so authors should
be careful not to word titles as if an error has necessarily occurred. 

Comment:
The lack of a keyboard accessible mechanism for displaying title content within all major UAs prevents keyboard users from accessing the pattern description.
The statement "Otherwise, assistive technology would be impaired" fails to address the actual accessibility implications. 

The use of the title, implied in the second paragraph above, as text for an error message, implies processing by the UA or assistive technology, and is
in effect a special casing of the title attribute. 

The definition of the placeholder attribute also contradicts the recommendation of using title.