I'll never understand why people insist on making tools that depend on non-standard/illegal HTML when HTML is perfectly extensible. Authors are free to invent c

I'll never understand why people insist on making tools that depend on non-standard/illegal HTML when HTML is perfectly extensible. Authors are free to invent custom tag names (according to some rules) and free to create attributes (according to some rules) but this appears to break those rules and depends on people authoring invalid illegal HTML. Why can't we extend standards-based languages using the built-in custom extensibility they already have?