The site says "Not supported in HTML5", but that is not a correct term (either). It should be: "Not valid in HTML5" or "Deprecated in HTML5". If you have the following code validated, you will see that it throws two errors and a warning:
Originally Posted by alpha91
<table border="1" cellpadding="5" cellspacing="5">
Browsers are in principle forgiving when it comes to deprecated HTML because of commercial reasons. But be aware: they are not when it comes to other reasons for invalidity, with only very few exceptions. So it would be wise to learn how to write valid code from the start. Also, if you are planning on doing web design for a living, writing valid code is good for PR and sales.
Lastly, writing valid code makes debugging it in case of problems much, much, easier.