There are two different aspects here:

• For HTML tags, you should only use those standard W3C HTML or XHTML tags (not those obsoleted ones), not tags that are only defined and supported by particular browsers. There is really no point to waste time, and deal with them. None of them makes your web page much better.
• For JavaScript, deal with the differences on client side, let your JavaScript handle it. JavaScript can detect the browser type, and for different types, you might need different syntax (this is life), and also some browser do not support certain things.

In general, it is not a good idea to send out different pages from server side, simply for different browsers.

Update:

Thanks for ViceRaid to follow up and elaborate on my thoughts.

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