Hypertext Markup Language (HTML):
Check on (X)HTML5 requirements and get warnings:
https://html5.validator.nu
Check on HTML for backward compatibility:
http://www.freeformatter.com/html-validator.html

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

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

Disable browser caching:
meta-cache-control

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webbrowser Firefox offers warnings for outdated functions
webbrowser Chrome offers warning for an outdated certificate