ValidationProblems

Top Ten MarkUp Validation Problems

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Todo

point to Tidy in the first paragraph
each problem should have associated sample documents and links to the validator output
of course, each problem should show the invalid markup
maybe the validator output should be contained inline
point to the "verbose" feature
links to validator docs
link to CSS Validator FAQ
check what to link and what not

Missing encoding information

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Missing document type declaration

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Missing "alt" and "type" attributes

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http://ppewww.ph.gla.ac.uk/%7Eflavell/alt/alt-text.html

Ampersands ("&") in links

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Improper nesting of elements

Proprietary elements and attributes

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embed
body @leftmargin, @topmargin, @marginheight, @marginwidth
frameset @border, @frameborder, @framespacing
table @height

...

Markup in document.write()

The <script> and <style> elements in HTML 4 are very special elements. Here is an example:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
  <title>...</title>
  <script type="text/javascript">
    alert("Björn");
  </script>
</head>
<body>
  <p>...</p>
</body>
</html>

If you load the document in a browser that supports scripting, it would show you a message box containing "Björn" rather than "BjÃ¶rn". This is because the browser ignores markup inside the <script> element. Here is another example:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
  <title>...</title>
  <script type="text/javascript">
    alert("Comments are <!--NOT--> ignored!");
  </script>
</head>
<body>
  <p>...</p>
</body>
</html>

You will get a message box containing "Comments are ignored!" for the same reason, markup is ignored. But the browser somehow needs to know where the <script> element is finished. It was unfortunately not possible to specify that everything up to </script> is considered part of the script, instead the first occurence of </ followed by a letter A-Za-z determines the end of the element. Here is an (illegal!) example:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
  <title>...</title>
</head>
<body>
  <script type="text/javascript">
    document.write("<p>...</p>");
  </script>
</body>
</html>

In this case, the first occurence of </ followed by a letter would be

! If you ignore the document.write() and ignore the

(remember, markup is ignored inside the <script> element) you get something like

  <script type="text/javascript">...</p>");
  </script>

And

does not match </script>, the validator complains

end tag for element "P" which is not open

It isn't, is it? Remember that comments are ignored too, but let's have a look at them again

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
  <title>...</title>
</head>
<body>
  <script type="text/javascript"><!--
    alert("--> BUH!");
  //--></script>
</body>
</html>

The browser would present you a message box containing "--> BUH!". If it had ignored comments you would not see the "BUH!", would you? But let's do another (illegal!) example

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
  <title>...</title>
</head>
<body>
  <script type="text/javascript"><!--
    document.write("<p>...</p>");
  //--></script>
</body>
</html>

So the </script>

</nowiki>

And

does not match </script>. As before.

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Confusing HTML and XHTML syntax

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http://www.w3.org/TR/xhtml1/#diffs

XHTML means lower-case

@@ (<META>, onMouseOver, etc.)

"non SGML" characters

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http://www.w3.org/International/questions/qa-controls.html