IDs Are Unique Per Page
IDs are supposed to be unique on a per-page basis. So HTML allows it to be defined universally because that's how it's supposed to be used. You use it as a site-wide identifier for portions of your page.
Many Browsers Will Display Broken or Bad HTMLMost browsers are forgiving and will allow lots of broken and invalid code. So, you can build tables and leave off all the end tags (/TD, /TR, /TABLE) and most browsers will still attempt to render the page. And in most cases will get the table right. But that doesn't mean that the page will validate or be viewed correctly in all browsers.
Posted in: CSS