If you cannot be bothered to Validate your code - how the hell can you expect others to test your site?
For those of you that don't know...
http://validator.w3.org
http://www.validome.org/
Fixing your problems will go a long way to ensure the site performs as you want!

You can chage the size of text in your Browser. Does it make the text on your design change size?
When it resizes, does it break the design?

Make sure that the font-face/family is legible and common. Text for PRINT should be in Serif...text for Screen, in large blocks, should be Sans-Serif as it is more readable!
Additionally... make sure the the Text Colour and the Background Colour stand apart... trying to read light grey on dark grey is pathetic... and believe it or not, most of us do not want to read red on yellow either!

Any Link should indicate what it will do, it should bear some relation to what the destination is!

When hovering over a Link, there should be a Tool-tip/Pop-up providing a little more detail on the link. This is not only to aid Users, but also considered more Accessible also.

Any Link should be distinct from the text around it - otherwise they will never know it;s a link. You can do this by leavig it underscored, making it bold, adding a background colour etc.

When a user hovers over the link, there should be something happening to let them know! Change of colour is always good... be it the text colour, the backgroud colour or both... the inclusion/removal of the underscore etc.
Also, please take the effort to include Active and Focus as well as Hover - as this means those using the Keyboard can get the same benefit!

Beleive it or not, Win IE is not the only Browser.
Test as you build - it will save time, effort and using a large number of swear words later!!!

Resize the Browser Window!
Make sure that making it smaller will not result in ugly things happening - this is usually easy to prevent, without major problems!
Always go for the lowest comon denominator... so aim for 800 wide as the minimum (thats 776px wide!).
Don't just shift the Width, alter the Height as well - if you have not used Positionig correctly, odd things will happen!

If they are not enabled, then people will not beable to sue your site!
Not all people have those things - particualry workbased machines!
Additionally - things such as Screen readers and Text-only tend to ignore Scripts... so again you lose!

Make sure you have a "fallback" in place. If the JS for IE will not be loading for IE... what will happen? Provide an alternative setup for people to navigate by.
If news is present in a marque or scroll layer - ensure that a block will appear isntead listing the news etc.