The documentation for global attributes contains the following text about the title attribute (<http://dev.w3.org/html5/spec/global-attributes.html#the-title-attribute>):
> Some elements, such as link, abbr, and input, define additional semantics for the title attribute beyond the semantics described above.
However, the documentation for the input element (<http://dev.w3.org/html5/spec/the-input-element.html>) makes no mention of what these additional semantics might be.
This was noted in the mailing list without any response: <http://lists.w3.org/Archives/Public/public-html-comments/2012Jan/0007.html>.
On the "common input element attributes" documentation (<http://dev.w3.org/html5/spec/common-input-element-attributes.html>) there are some references to input title semantics:
> When a control has a pattern attribute, the title attribute, if used, must describe the pattern.
> 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.
> The placeholder attribute should not be used as an alternative to a label. For a longer hint or other advisory text, the title attribute is more appropriate.
> Note: These mechanisms are very similar but subtly different: the hint given by the control's label is shown at all times; the short hint given in the placeholder attribute is shown before the user enters a value; and the hint in the title attribute is shown when the user requests further help.
But this information should be generalized/consolidated and divulged on the main documentation for the input element, especially since the natural thing to do when trying to determine what the additional semantics are is to click on the "input" link from the global title attribute documentation.
The documentation for the abbr element (<http://dev.w3.org/html5/spec/the-abbr-element.html>) and the link element (<http://dev.w3.org/html5/spec/the-link-element.html>) both contain notes that say "Also, the title attribute has special semantics on this element" and in the descriptions those special semantics are elaborated. Input should probably follow this same model.
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Change Description: https://github.com/w3c/html/commit/2d5dfcef7d53d745bc75c0797117bf618c4326b0
Rationale: Since this bug was filed, the "Also, the title attribute has
special semantics on this element" text was added to <input>, and the
section defining pattern="" describes the additional semantics. In
2d5dfce I've updated that sentence to point the reader at pattern="" to
make this more discoverable. You can see this change in the 5.1 Nightly
If this change satisfies you, Matt, I'll also merge it into the 5.0 spec.
This looks great! Thank you, Edward.