I think every web developer has used a markup validation service at least once. Well, unless you've only been messing around with certain content management systems and heavy server technologies like ASP.NET WebForms which leave you little chance to produce valid markup. And I think every developer who has used an online validation service has wished he had an offline tool for that.
Normally, developers feed their pages to the W3C Markup Validation Service. It lets you validate you (X)HTML markup, CSS stylesheets, RSS and ATOM feeds and also has other tools in the box. As great as it may be, it's not the most convenient way to check your work. For starters, it requires an Internet connection which you may not always have (if you're somewhere on holidays or on the country side). Even if you have a network access around the clock, you know that this service is often down. It also has restrictions which stop bulk-checks to minimize their traffic consumption. So you're stuck with manual checks with an unreliable online service. Naturally, you wish to have an offline tool right at your finger tips whenever you need it.
In fact, you can download the source code for the W3C validator and run it on your local machine. The usage is though not quite straightforward. First, the thing is offered as Perl software running on a web server, so you need the skills to install and configure Apache server. That may not be a five-minute-task for those who haven't worked with it before (like us, ASP.NET guys). I wish they could offer it as a simple utility which would read from a URL and display the results in a window. Even better if that utility could be integrated with popular browsers.
Now to the browsers. There are a few plugins for Firefox which provide validation functionality. For instance, the now deprecated Offline Page Validator which submits a page to the W3C service and displays the returned results. Since it is again dependent on the W3C online service, the thing is of little interest.
There are offline validation plugins like the Html Validator and the Total Validator. Even so, these are offered by third parties which is kind of unreliable since the team may decide to drop the project one day (a few abandoned validator plugins are already out there). I'd prefer some sort of "official" and supported validation feature.
With the other browsers it is even worse. For Opera I could only find some simplistic Site validator widget. For Internet Explorer I didn't find anything. For Safari I've seen the Safari Tidy plugin which seems to be dependent on the W3C online validator. For Chrome there is the HTML Validator plugin which again seems to be dependent on the W3C online service.
Needless to say, I'm depressed with that state of the things. Why can't browser builders add validation directly to their products? There is anyway a document parser built-in, so it only takes to publish its output with some user-friendly messages. It can't be that complex. And it is very much needed.
Some say a browser is only for viewing things. I don't agree. It may look like nothing but an end-user software utility. But it is also the first tool in a web developer's workshop. Developers test their work in those browsers so they partially perform a function of a development environment. If so then browsers should also have some functions important for developers. Validation would be the first one to add.
Others say it would be of disadvantage for site owners if browsers scared their users with a pile of error messages. Again, I disagree. For once, this validation function could be made hidden in developer menus where a normal user would not normally go. And even if he did, it may actually be a good thing. A number of repelled users should provide sufficient motivation for developers to check their work. I can't say how many times I've seen a site full of errors of a company offering products for businesses and promising professionalism, unprecedented quality and highest data security. Kind of funny. If you can't even make your web pages validate, what could possibly convince me you excel at the rest of your software?