Because some features in earlier versions of IE arenít compatible with HTML5 parsing, weíve removed them from IE10 mode. Sites that rely on these legacy features will still work when running in legacy modes. This way, sites that work today will continue to work with IE10 even if the developers of the site donít have the time to update them.
This content is ignored in IE10 and other browsers.
In older versions of IE it renders as part of the page.
This means conditional comments can still be used, but will only target older versions of IE. If you need to distinguish between more recent browsers, use feature detection instead.
also removing support for Element Behaviours and XML Data Islands
Inquiring minds want to know: Does this mean that MSIE 10 will be taking the unheard-of step of doing all the things w3c says "compliant user agents" are supposed to do*? Its failure to do so was the main reason anyone ever needed to use the Conditional Comment.
* To say nothing of the things other browsers had figured out by 2004, like font substitution.
Because you know, it's not like in 2020 IE9's CSS will be substandard or it's lack of support for the video element will be anything we'll have to deal with. Oh wait, we will have to deal with it. Here's an idea, keep moving stranglers away from Internet Explorer.
doing all the things w3c says "compliant user agents" are supposed to do*?
yes, I do :) - I think it's a positive step, taking away the "pacifier" and forcing themselves to comply properly, I read they actually recommend using the HTML5 Doctype (forgot the page now, though it was in the area of their rendering modes, which is a nightmare scenario!) - so if they are recommending that along with the withdrawal of legacy features that will affect parsing and validation then I can only assume they now have it right - finally :)*
* one can hope after all this time, though since 8 they have been coming on leaps and bounds