Pixel Pete saysXHTML5 is HTML5 written in strict XML syntax — the modern way to use XHTML.
HTML5 has two serializations: the usual HTML syntax, and an XML one called XHTML5 (HTML5 served as application/xhtml+xml). It uses the simple <!doctype html>, the XHTML namespace, and strict XML rules. A polyglot document is written to be valid as both — useful when an XML toolchain must consume your pages.
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XHTML5 = HTML5 in XML serialization
Simple doctype + XHTML namespace + XML rules
Served as application/xhtml+xml
Polyglot docs are valid as both
Quirks Mode attacks — common mistakes
Confusing XHTML5 with old XHTML 1.x
Forgetting the XML MIME type requirement
Writing non-polyglot markup when both modes are needed
Boss battleNote the two changes that turn an HTML5 page into valid XHTML5.
Example code
<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>HTML5 → <!doctype html>, served text/html
XHTML5 → same + xmlns + self-closed voids,
served application/xhtml+xml</pre></body></html>