A web site must be accessible and pleasant to view by most web users. My web pages are always tested to work at the minimum in the following environments:

Browsers

Internet Explorer 7

Internet Explorer 6

Firefox 2.0 and 3.0

Opera 9

Safari 3.1

(together these represent 99.3% of web users)

JavaScript

I recently expanded my use of JS to provide a more complete user experience on my websites; I still strongly believe in graceful degradation and layering of features. Any JS that I use is the icing on the cake, not the main course.

Screen Resolution

I use fluid layouts whenever possible to ensure that users with screens at low resolution will never have to scroll right, while users with high resolution view a comfortably wide web page.

Screen resolutions:800×600,1024×768, larger (the number of people still browsing at 640×480 is extremely small, under 0.1%).

Font size

Many web users will scale up or down the default text size in their websites. I try to make sure most items will scale comparably and that the site will be readable at least one step up and one step down.

Compliance and validation

I do not use obscure hacks for Internet Explorer, instead I use conditional comments (which are valid).