Online Validators Revisited
Some time ago, I did a comparative review of the (then) current online validation services. Things have changed considerably since that time.
I've already mentioned this above. This is THE validator because it is run by the people who make the rules. Whichever other validator you might want to try, use this one first to check your HTML and CSS. That is all it does, it won't tell you if your page size is wrong or if your images are too big or if the type is too small. Think of it as a structural test to make sure that your pages stand up in the wind of the Web. Free.
Bobby checks your HTML against real world browsers. This was a good idea at one time but it allows sloppy, old code to slip through that really should be weeded out and replaced by W3C compliant HTML. There is also a check for the accessibility of your page for people with disabilities. For certain kinds of sites including many governmental and educational ones, this is mandatory, not just optional. Free.
WebSite Garage was once one of the better validators around. It has now been adopted by Netscape and sometimes I find that it is difficult to get through. It does something called a "Tune Up" that check your pages for browser HTML compatibility (same problem as Bobby in that respect) but it also reports on the page load time, dead links, link popularity, spelling and how well your page will be indexed by search engines. So, it is a very useful facility after you are satisfied that the HTML is W3C compliant. Free.
This is the only validator that you have to pay for. You can have a one-off report for $15 or take out a subscription for $135 per year per domain – that could add-up pretty quickly if you do a lot of different sites. This is the only one that will show you what your page looks like in different browsers, at different screen sizes and computer platforms. It actually renders the page and displays an image. It also reports on errors made by your HTML editor and knows about the bugs in the various browsers. Have a look at the Quick Tour and decide if it would be of use to you. I prefer to have the actual computers, screens and browsers on my desk so that I can test them myself, but that is expensive too.
I can't really give these sites scores on a comparative basis because they do quite different jobs. They are all free to use except Net Mechanic. Use them!