I recently added HTML5
<footer> elements to a page, thinking that HTML5-capable browsers would use them for semantic things like navigation, while older browsers would see them as user-defined tags and apply CSS styles to them like ordinary
When I was working on this, I remember seeing a warning that a version or some versions of Internet Explorer didn't support user-defined tags, with a link to HTML5shiv as a possible solution. Unfortunately I don't remember where I saw it or which version/versions might be affected (which would indicate how concerned I should be about this problem).
If other common browsers share this problem, it would help to know about those too.