The purpose of this page is to enable collaborative creation of brief advisory text for each HTML5 microsyntax so that when an attribute value does not conform to a given microsyntax, a validator can display the advisory text about the syntax to guide the author to fix the value. Note that like the rest of this wiki, editing requires you to agree to release your contributions under the MIT license (see wiki footer). Please note that while copying text from the WHATWG version of HTML 5 is OK, copying text from RFCs or W3C specs is not OK.
Note that some formats pertain to Web Forms 2.0 (e.g.
Please keep descriptions short: one paragraph in length.
A browsing context name is any string that does not start with a U+005F LOW LINE character (
_), or, a string that case-insensitively matches one of:
(time element attribute version)
(time element text content version)
An ID consists of at least one character but must not contain any whitespace.
An integer consists of one of more characters in the range U+0030 DIGIT ZERO (
0) to U+0039 DIGIT NINE (
9), optionally prefixed with a U+002D HYPHEN-MINUS (
An RFC 4646 language tag consists of hyphen-separated ASCII-alphanumeric subtags. There is a primary tag identifying a natural language by its shortest ISO 639 language code (e.g.
en for English) and zero or more additional subtags adding precision. The most common additional subtag type is a region subtag which most commonly is a two-letter ISO 3166 country code (e.g.
GB for the United Kingdom). IANA maintains a registry of permissible subtags.