Joomla! Documentation™

Browsers and other devices that reads HTML and display them are called HTML parsers. These parsers all work based on a set of instructions and these instructions are built based on certain rules known as standards. If you do not follow the standards, this means you are doing something that is not covered by the instructions of the parsers and the result is the parsers displaying something other than what you expect. They are not able to determine how to process the page correctly.

There are, of course, vendor-limited standards such as the use of ActiveX, but as these standards are not widely supported, many parsers would not be able to process the page correctly. This means that many people would not be able to see your page as you intend it to be seen.