Bad tags usually come down to tags that are presentational, which is the realm of CSS, or simply not valid, leading to unreliable code that can’t be guaranteed to work on different or future browsers.
With HTML pages on the web you can just include the required Javascript between tags. When you validate the HTML on your web page the Javascript content is considered to be CDATA (character data) that is therefore ignored by the validator. The same is not true if you follow the more recent XHTML standards in setting up your web page. With XHTML the code between the script tags is considered to be PCDATA (parsed character data) which is therefore processed by the validator.
Because of this, you can't just include Javascript between the script tags on your page without 'breaking' your web page (at least as far as the validator is concerned).
