I disagree. Because of my background as a programmer, it never even occurred to me to mock up websites using Photoshop when I started--it was 100% CSS+HTML, all the time. But, a few times I was designing something and had to to show a colleague a draft in a hurry, so I did a screenshot of the incomplete page, pasted into MS Paint, and embellished it there. When they made suggestions, I just went back into MS paint and re-arranged things again in a few seconds and resent. It was amazingly productive compared to writing all the code. And, I didn't even have layers--it was MS Paint!

After a few experiences like that, I switched to a hybrid, iterative approach where I would mock stuff a few aspects of the page in a real image editor and then code it when I found the correct look. It really saves a lot of time when I am doing anything that requires non-trivial CSS and HTML.

It saves time because I end up throwing away the majority of my designs after I see them--they look better in my head than they do on the screen. If I just did it in HTML+CSS then I'd waste more time just doing the coding and I've have less time to experiment with different looks.

The thing to keep in mind is that it isn't necessary to spend hours and hours getting everything (e.g. font anti-aliasing, color shades, spacing) pixel-perfect in Photoshop. You just need to get it good enough where you can tell that the design is going to work. Then you can code it up as a HTML+CSS mockup. No matter what you do, the Photoshop mock-up and the final version aren't going to look the same anyway (especially when you consider browser differences).