Is it OK to use unknown HTML tags?

Question

Correct me if I\'m mistaken, but AFAIK, unknown HTML tags in markup (i.e. tags not defined in the HTML spec, like say, ) will eventually be treate

Answer 1

You should always approach HTML as it is defined in its respective specification. "Defining" new tags is a bit of an extreme approach. It might pass a browser check because it implements various failsafes, but there is no guarantee of this. You're entering the land of Undefined Behaviour, at best. Not to mention you will fail validation tests, but you seem to be aware of that.

If you wish to be more semantically expressive in your markup, you can use HTML5 which defines quite a bit of more descriptive tags for describing the structure of your page instead of generic divs which need to be appended ids or classes.

In the end, a short answer: No, it's bad practice, you shouldn't do it and there could be unforeseen problems later on in your development.