Unfortunately, the default settings aren't ideally suited to this purpose. I've found that zooming is much more useful if you set your options as shown in this image. First and foremost, you'll want to turn off the Smooth Images option, so you can see your enlarged pixels in their natural state. Second, you may want to turn off Zoom Follows Keyboard Focus -- this allows you to stay zoomed on one spot, regardless of what's happening on-screen. This is especially useful when switching tabs in Safari to compare two versions of the same page while zoomed in.
The most important (and least intuitive) change is to set the maximum zoom to 2x, using the topmost slider. With this setting, your initial zoom will jump straight to 2x, skipping the seven steps between 1x and 2x. These seven levels aren't very useful to a web developer, as the pixels become distorted and weird looking since they're being magnified by some non-whole number. Once you jump to 2x, you can continue zooming in for 3x, 4x, etc.
The one downside of this zoom setting is that you can't zoom back out in increments -- it jumps all the way out to normal magnification. But for me, this is actually useful, as when I'm done zooming, it's easier to jump out of zoom with one keypress.
Try it -- you'll like it!

