I am convinced the one single thing that made HTML unusable over the time was that people wanted or needed a way to re-use parts of the page across multiple pages, like headers, navigational elements and footers.
This meant people used frames, PHP, templating engines or any other new technology mainly for the purpose of creating shared elements, simply because HTML failed (and to this day: fails) to offer a way to include one HTML file in another without having it suck (like frames definitely did, since the browser treated each subpart of the page like its own entity including caching).
