Is it compulsory to start with <!DOCTYPE>

Answer 1

No, it's not, but it's best practice to do so for various reasons (like ensuring the browser reads the page in a predictable manner, search engine optimization, etc.)
On a different note...
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Happy coding!
P.S. Please see the following post for a humorous interpretation (but still an earnest plea).
https://www.sololearn.com/post/10362/?ref=app

Answer 2

Its not compulsary.
But if you want to use Html5 features , you have to write it in your document.
Writting it ,it tells the browser that your website is using latest html5 so it renders your website accordingly.
Also if this helps☺️☺️
https://www.sololearn.com/Discuss/1444902/?ref=app