Why is my site’s markup polygot XHTML5? I have had to deal with some really awful user-agents:
- Bespoke markup parsers in RSS readers.
- Link previews in obscure messaging apps.
- A reader-mode bookmarklet-turned-browser-extension that hasn’t been updated in twelve years.
- Various search engines trying to parse the page without using a compliant parser.
Most of my issues were solved by running my generated markup through both xmllint (XML syntax) and the Nu HTML Checker (HTML5). Optional elements tend to cause issues the most.
Overly-aggressive validation tends to spot latent bugs. Even if something is valid without an optional closing tag, I may have meant to include one in my templates. If that becomes an issue when I change something else later, it’ll be hard to track down the bug when it passes more lax validation.