.xhtml

XHTML web-page markup, XML-based HTML rendered by web browsers

Syntax highlighted

Format facts

MIME type: application/xhtml+xml
Apple UTI: public.xhtml
Stock Quick Look: shown as plain text
Planned approach (census): syntax-highlight
Icon hint: renders
Prevalence: common

Renderer note (from the census)

macOS renders XHTML natively

Gotchas: strict HTML