Steps to reproduce:
choose in the browser interface the "Alternate stylesheet" so that the next line can become orange. E.g. like View/Style/Alternate stylesheet
This absence of support has been reported for now 10 years by webstandards.org in its IE 5 Top 10 CSS Problems.
section 1.1 states
"The 'LINK' element references alternative style sheets that the reader can select".
section 3.2 UA Conformance states
"5. If the source document comes with alternate style sheet sets (such as with the 'alternate' keyword in HTML 4), [then] the UA must allow the user to select which style sheet set the UA should apply."
4.01, section 14.1 states
"User agents should give users the opportunity to select from among alternate style sheets or to switch off style sheets altogether."
4.01, section 14.3.1 and section 14.3.2 states
"Authors may specify a number of mutually exclusive style sheets called alternate style sheets. Users may select their favorite among these depending on their preferences. (...) User agents should provide a means for users to view and pick from the list of alternate styles."
User Agent Accessibility Guidelines 1.0, guideline 4.14, provision 1
"Allow the user to choose from and apply alternative author style sheets (such as linked style sheets)."
Apply alternative author style sheets test from WAI and the UAAG test suite
In his November 2nd 2001 article Alternative
Style: Working With Alternate Style Sheets, Paul Sowden states
"(...) But then we encounter a problem. A major one. Mozilla provides a menu to select the style sheet we want to use under the view menu item. But Microsoft Internet Explorer (MSIE) provides no such menu. So we have several style sheets, and no way to access them in MSIE."
This issue has also been reported by Mark "Tarquin" Wilton-Jones in his No, Internet Explorer did not handle it properly: Alternate Stylesheets
Firefox 1.5, Firefox 188.8.131.52, Firefox 3, Opera 9.27, Opera 9.50, Konqueror 3.5.8, Seamonkey 1.x, Seamonkey 2.x, NS 7.2, Galeon 2, Epiphany 2 all pass this test.
Update: This bug has been FIXED in Internet Explorer 8 beta 2 (build 6001.18241) released on August 28th 2008. Otherwise, Internet Explorer 8 beta 2 PASSES this specific and particular testcase.