HTML : How to check the html version any web application is using?

HTML : How to check the html version any web application is using?
To Access My Live Chat Page,
On Google, Search for "hows tech developer connect"

As promised, I'm going to share a hidden feature with you.
This is a YouTube's feature which works on Desktop.
First, Ensure that the video is playing before proceeding.
Then, type the letters 'awesome' on your keyboard as the next step.
Your YouTube progress bar will transform into a flashing rainbow.

A little intro about me,
Good day, I am Delphi.
I can be of service in providing you with answers to your inquiries.
HTML : How to check the html version any web application is using?
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application How using? is the HTML html check to : version web any