behavior of browsers for css application when multiple elements share same id

Question

I understand that IDs are supposed to be unique in HTML documents. But a lot of HTML pages are sloppy and don\'t follow the rule. My question is: how do browsers handle CSS

Answer 1

For an example of unintended consequences, see this question I answered:

Yet another IE 'Object expected' error with no information

You have this HTML, once for each tab:

    
Introduction

The problem is that you're specifying two elements with id='introduction'.

For various reasons, you should not do that:

• It's causing validation errors.

  Line 37, Column 27: Duplicate ID introduction.
  Line 36, Column 39: The first occurrence of ID introduction was here.

• It's breaking your tabs in IE7.