How to use custom html tags?

Question

JavaScript

I want to use a custom tag in html5, in practice this usually doesn't cause any problems.
All browsers usually understand everything, styles are applied, js works.
But what problems can still be with this?
Everything I can't google always comes down to some ancient attempts to use tags instead of classes, something like a font20 tag that sets the font size and other mind-blowing things. Advisers usually send the questioner there to the classes and that's it.
Sometimes there are tips to make an insert at the beginning of the js document that creates a tag:
document.createElement('tagname');
and only then use it in the document. If necessary?
Will native tags cause problems in mobile browsers?
Will there be problems when viewing the site not through a browser, but with the help of alternative programs like Internet Explorer or the like?

Answer 1

Separate presentation of data from styling
At least < ie9 will have problems and not only
caniuse.com/#search=Custom%20Elements
stackoverflow.com/questions/2802687/is-there-a-way...