TYPO3 Core

Request declined because it violates HTML specification, namely part 12.4, which says:

12.4 Path information: the BASE element
<!ELEMENT BASE - O EMPTY               -- document base URI -->
<!ATTLIST BASE
  href        %URI;          #REQUIRED -- URI that acts as base URI --
  >

Start tag: required, <b><u>End tag: forbidden</u></b>

Reference: http://www.w3.org/TR/html401/struct/links.html#h-12.4

Then Typo3 currently violates exactly this rule.
It generates the base tag as <base /> which means that the base tag
has an end tag (following xml spec
http://www.w3.org/TR/2006/REC-xml-20060816/#sec-starttags) although
the element is empty. And empty elements can be written with an <tag/>
or <tag></tag> following above spec.

For strict html 4.01 compliance the base tag had to be <base href="...">
without any slashes.

Nope. Closing tag means exactly what you propose: </base>. But <base ... /> is allowed. See here: http://www.w3.org/TR/xhtml1/#C_2

Do not refer to xml while working with html. This is not the same.

See: CSS-bugs with IE 6 http://drupal.org/node/44934

<base href="http://example.org/"><!--[if IE]></base><![endif]-->

I had the same problem on IE6 with googlemaps, that couldn't be displayed anymore.
In fact, I had an error like that:
"Internet Explorer couldn't open the site
http://...
Operation aborted"

After some researches, I discovered this problem on googlemap forum:
http://groups.google.com/group/Google-Maps-API/browse_thread/thread/3b9f5d5bf103a02b/e4f2cad7b366ad78?lnk=gst&q=IE6+operation+aborted#e4f2cad7b366ad78

It is a problem due to the tags closed uncorrectly, and more specifically the tag <base>.

So, I don't know if there is already a patch for this problem, but you can see here my solution:

1). class "tslib_pagegen.php": line 488

I replaced this line:
$GLOBALS['TSFE']->content.='
<base href="'.htmlspecialchars($GLOBALS['TSFE']->baseUrl).'" />';*/

with this one
$GLOBALS['TSFE']->content.='
<base href="'.htmlspecialchars($GLOBALS['TSFE']->baseUrl).'"></base>';

2). class "tslib_fe.php": line 1393

I replaced this line:
$replacement = chr(10) . '<base href="' . htmlentities($base) . '" />' . chr(10);
with this one
$replacement = chr(10) . '<base href="' . htmlentities($base) . '" ></base>' . chr(10);

I hope this help

Sorry, guys, we can't violate the spec because of a bug in the old browser.

Use can TS conditions to solve the problem:

[browser = msie][version < 7]
page.headerData.100 = TEXT
page.headerData.100 = <base href="http://example.com/&quot;&gt;&lt;/base>
[else]
config.baseTag = http://example.com/
[global]