All browsers know how to deal with correct HTML. However, if
it is incorrect, the browser has to repair the document, and
since not all browsers repair documents in the same way,
this introduces differences, so that your document may look
and work differently on different browsers. Since there are
hundreds of different browsers, and more coming all the time
(not only on PCs, but also on PDAs, mobile phones,
televisions, printers, even refrigerators), it is impossible
to test your document on every browser. If you use incorrect
HTML and your document doesn't work on a particular browser,
it is your fault; if you use correct HTML and it doesn't
work, it is a bug in the browser.