the spec suggests user agents use the title attribute for description strings that may be displayed at any time. Due to user agents not implementing 'tooltips' accessible the content is not available to keyboard only users until a incorrect pattern has been put in.
"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."
suggest adding authoring advice to mitigate this for example:
"authors should not rely on this method, also provide a description of the pattern in text so that all users can have access to the information"
This bug was cloned to create bug 17939 as part of operation convergence.
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Change Description: added advice to http://www.w3.org/html/wg/drafts/html/master/forms.html#attr-input-pattern
Rationale: the spec did not match reality in its advice, it does now.
refer to http://lists.w3.org/Archives/Public/public-html/2013Mar/0118.html for pointer to HTML WG discussion.