I understand that with a few simple arrangements, the new HTML5 elements can be used even in older browsers. Does this mean the browsers simply don't really care about what the tags are named? So would there be any critical issues with making up your own elements, like <comments></comments> or <feature></feature> or <chapter></chapter> or whatever, instead of using id's and classes?
I realize it's pretty stupid and unnecessary, but from a hypothetical point of view, would it be likely to cause any actual problems?