XSLT solved this problem. But it had poor tool support (DreamWeaver etc) and a bunch of anti-XML sentiment I assume as blowback from capital-E Enterprise stacks going insane with XML for everything.
XSLT did exactly what HTML includes could do and more. The user agent could cache stylesheets or if it wanted override a linked stylesheet (like with CSS) and transform the raw data any way it wanted.
XSLT did exactly what HTML includes could do and more. The user agent could cache stylesheets or if it wanted override a linked stylesheet (like with CSS) and transform the raw data any way it wanted.