Next section: HTML and SGML
Up to main index

[see parent document for copyright information]

Section 1: KGV and other validators

What does KGV do?

The Kinder, Gentler Validator (hereinafter "KGV") is sort of like lint for C. It compares your HTML document to the defined syntax of HTML and reports any discrepancies.

Why should I validate my HTML pages?

One of the important maxims of computer programming is:

Be conservative in what you produce; be liberal in what you accept.

Browsers follow the second half of this maxim by accepting Web pages and trying to display them even if they're not legal HTML. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal construct; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a mangled mess, or even crash.

That's why you want to follow the first half of the maxim by making sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validators.

What other validators are there?

Besides KGV, the three most commonly mentioned HTML validators are:

     Weblint: http://www.cre.canon.co.uk/~neilb/weblint/
    HTMLChek: http://uts.cc.utexas.edu/~churchh/htmlchek.html
    WebTechs: http://www.webtechs.com/html-val-svc/
        (née HALSoft)

Weblint and HTMLChek are heuristic validators --- that is, they do not completely parse your HTML markup, but simply scan it looking for errors. The advantage of this is that they can detect constructs that are legal HTML but considered "bad style", such as an <IMG> tag without an ALT attribute; the disadvantage is that they can fail to detect some errors. WebTechs is similar to KGV; both operate directly from the HTML language definition, and both strictly obey the rules of SGML. If your document passes one of these validators, you know it's clean.

I recommend using a combination of validators: one of Weblint or HTMLChek, and one of WebTechs or KGV. Each has features that the others don't, and they complement each other nicely.

How does KGV work?

KGV is based on James Clark's nsgmls SGML parser. The Validator itself is a CGI script that fetches your URL, passes it through nsgmls, and post-processes the resulting error list for easier reading.

What do the options on the KGV main page do?

In addition to running your document through nsgmls, KGV can perform other checks and transformations on your document:

• The "Include Weblint results" option runs your code through Weblint, as described above in the section on other validators. The "run Weblint in `pedantic' mode" sub-option increases Weblint's `pickiness'.
• The "Show source input" option includes the contents of your document in KGV's final report. This is handy for making sure that KGV is validating the document you think it's validating.
• The "Show an outline of this document" option uses the <Hn> elements in your document to construct an outline or "table of contents" of the document. If you've used the <Hn> elements for something other than section headers (for instance, to get fake font changes), this outline will probably come out very strange.
• The "Show parse tree" option is an experimental feature which uses the output of nsgmls to construct a neatly formatted parse tree of the HTML markup in your document. The exact format of the resulting parse tree is undergoing revision.

  You may notice that the parse tree contains HTML tags which don't appear in your document --- end tags for P and LI elements, for instance, or start and end tags for the HTML, HEAD and BODY elements. This is a reflection of how SGML parsers operate. The tags in question are marked as optional in the HTML DTD; the author may therefore omit these tags, and the parser will infer their presence at the appropriate place. For instance, after a <LI> start tag, the parser will infer a </LI> end tag just before the first subsequent tag that cannot appear in a LI element --- typically the </UL> end tag or the start tag of the next LI element.

How do I set KGV to validate in HTML2/HTML3/Netscape mode?

Unlike WebTechs, KGV does not allow you to select a document type on the fly. If your document is not intended to be HTML 2.0-compliant, you must add a corresponding DOCTYPE declaration to your document; see the section on DOCTYPE for more information. If your document does not have a DOCTYPE declaration, KGV will assume an HTML 2.0 document type (and will tell you it is doing so).

Can I install a copy of KGV locally?

Apparently, you now can; see http://validator.w3.org/source/.

Help! KGV spewed a zillion error messages on my page!

Don't panic. Did KGV complain about your DOCTYPE declaration (or lack thereof)? Make sure your document has a syntactically correct DOCTYPE declaration, as described in section 2, and make sure it correctly identifies the type of HTML you're using. Then run it through KGV again; if you're lucky, you should get a lot fewer errors.

If this doesn't help, then you may be experiencing a cascade failure --- one error that gets KGV so confused that it can't make sense of the rest of your page. Try correcting the first few errors and running your page through KGV again.

I used the Netscape document type, but KGV still doesn't like my tables.

Unfortunately, this is one of the areas of HTML that Netscape botched pretty badly; some of Mozilla's behavior regarding tables simply cannot be represented in a DTD. The two thorny bits are:

• The BORDER attribute. Netscape allows this attribute to be used both as a binary flag (ie. <TABLE BORDER> to indicate the presence of a border) and as a width specifier (ie. <TABLE BORDER=5> to specify a five-pixel border). There's no way to write a DTD that can encompass both of these uses; WebTechs' Mozilla DTD, which KGV uses, allows the latter usage and not the former.
• The WIDTH attribute. Netscape allows the value of this attribute to be both absolute (ie. <TABLE WIDTH=50> specifies a width of 50 pixels) and relative (ie. <TABLE WIDTH=50%> specifies half the page width). KGV appears to have fixed this part of the WebTechs Mozilla DTD; if you put the attribute value in quotes, as noted above, this should come out fine.

I love your validator/I have a question about your validator.

Well, thanks, but it's not my validator. KGV is maintained by Gerald Oskoboiny (gerald@cs.ualberta.ca); questions or problems concerning the validation service itself should go to Gerald. This FAQ is maintained separately by me, Scott Bigham (dsb@killerbunnies.org); questions like, "This error isn't listed; what does it mean?" and "Are you sure this is illegal HTML?" should go to me.

Next section: HTML and SGML
Up to main index

[see parent document for copyright information]

Sending feedback? Check here first.

Last update 08 Aug 98