Is it possible to create custom tag like p or h1 in html5?

Answer 1

Since html is based on xml, yes. you are free to do :
<custom>123</custom>

and grab it in js too like document.getElementByTagName('custom')[0]
but might throw annoying console warnings if not registered through javascript code...

Answer 2

https://code.sololearn.com/WnmObBR1OFtw/?ref=app
https://code.sololearn.com/WMQA8yiYK8so/?ref=app

Answer 3

Just to save you future frustration I had.
To access the property values of a custom tag you need to use .getAttribute("name")