HTML 4 is not an XML language: it uses tags, but the concept of a document being structurally invalid is not the same as XHTML. Every browser that handles HTML

HTML 4 is not an XML language: it uses tags, but the concept of a document being structurally invalid is not the same as XHTML. Every browser that handles HTML 4 supports a kind of "tag soup". You must take explicit action to shift them into a strict mode; consequently, it does not take longer to "parse a broken schema" -- handling an HTML document without the final closing tags should be no slower than a complete document, because no validation is taking place.