– html “xmlns=http://www.w3.org/1999/xhtml” its namespace name is default, and will be added even if you do not include it;
– added the plugin from Chrome for HTML is an easy way to see HTML and check on warnings and errors.
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webbrowser Firefox offers warnings for outdated functions
webbrowser Chrome offers warning for an outdated certificate