Face it, we don't have a lot of control over typography on the web. We have about a half-dozen fonts to pick from, we don't know what size or exactly what color the type will be, and the exact same type will render very differently on a Mac than a PC. But the increasing browser support of the CSS text-shadow property is giving us more fine-grained control of text weight and contrast.
Text shadow has been supported in Safari since version 1.1 (as well as other Webkit-based browsers like Google Chrome) and in Opera since version 9.5. With the release of Firefox 3.5 last week, all the major browsers aside from Internet Explorer support shadows. Fortunately, since we will only be making subtle adjustments to our typography, it isn't a big deal even if the viewer is using a browser that doesn't support shadows. They will simply see the default look.
Normally, there are two font weights on the web: normal and bold (in theory there are a range of weights from 100 to 900, but there is little practical support for that). However, we can fake an intermediate text weight by setting the text-shadow color to the text color, and setting the blur to zero.
Normal text / Bold text / Text with
text-shadow: 0 0 0 #000;
We can also reverse the trick and make the text a bit lighter with nearly-transparent shadow with 1 pixel of blur behind the text. I first saw a similar trick on Wilson Miner's site, although he has since removed it. The version using alpha transparency was developed by Komodo Media. Unfortunately, this technique is a little less predictable in browsers other than Safari, particularly in the current version of Chrome which actually darkens the text.
This has no text-shadow / This has
text-shadow: 0 0 1px rgba(0,0,0,.01);
Local Contrast Enhancement
There is a trick commonly used in Photoshop to make photos "pop" with more contrast, but without modifying the overall tonal range of the photo. You can read more about local contrast enhancement elsewhere, but the essence of it is increasing the contrast of every edge in the photo—making the darker side a bit darker and the lighter side a bit lighter. We can use text-shadow to do the same thing with low contrast text.
Note: I am not advocating low-contrast text on your website! Make sure the text is readable even before you add any shadows.
So for instance if we have some brown text on a tan background, we would need to add a lighter tan "glow" around the text. That will increase the contrast immediately around the text itself, without changing the rest of the background.
No contrast adjustment / With
text-shadow: 0 0 5px #f1e2cd;
You can see that this method lightens the text-weight slightly (just as in the previous example) but it also makes the text look sharper. The trick here is to pick a shadow color that sets off the text without looking artificial. In particular, this can be useful when you are overlaying text on a background image: the slight glow will make the text easier to read.
With all of these techniques the trick is getting the right combination of shadow color and blur radius: keep experimenting until you get it looking the way you want. And make sure to browser test! We are using cutting-edge technology here, so it will look a little different from browser to browser.
Finally, there are undoubtedly many other interesting effects to be had by manipulating the text-shadow property. If you find others, please share them in the comments.