Is it a bad practice to add extra attributes to HTML elements?

Question

Sometimes I add an attribute to some of my controls. Like:

Chris Sharma

I know it

Answer 1

It's invalid XHTML which is a bad thing - mainly because you can't show off with valid ;) Every mainstream browser and search engine will ignore extra attributes happily though. You could add an extra namespace though to make your XHTML valid again.

  
  Chris Sharma
  

That's perfectly valid XHTML. However, W3C Validator will still refuse to validate it (I think). It's a shortcoming of their XML parser. For such non namespace aware parsers, my:isClimber will still be treated as would be isClimber. But you can now rest easy as you know that it is valid XML and finally that's what counts, isn't it ;)