Debug HTML coding

Hypertext Markup Language (HTML):
Check on (X)HTML5 requirements and get warnings:
Check on HTML for backward compatibility:

Note: html “xmlns=” is default, and will be added even if you do not include it.

Examples of solid XHTML for an email text:
tried-and-tested table layout
appointment at the dentist

Disable browser caching:

Control loop for a website (using F12 in Windows):
webbrowser Edge offers warnings on HTML5 requirements (and debug of script)
webbrowser Firefox offers warnings for outdated functions
webbrowser Chrome offers warning for an outdated certificate