The normally outputted code is valid xhtml 1.1 but certain functions are disabled for browsers other than IE through settings in your web.config file. Please don't say this is minor, cause it isn't (alternative browsers are said to be used by 10% of the
internet population and rising). You can either change the settings in your web.config file, or add clientTarget="ie5" to your page line in your aspx file if you're sure the code you've written will work on every browser. This will ensure that the correct
code is being output for all your clients.
You may argue about why microsoft decided to set these web.config settings as default (3 times because asp.net 1.0 and 1.1 also suffer from this) but I doubt that is within the scope of this topic