For the clientlibs i am including in the pages
ex: <cq:includeClientLib css="myproject.components" />
Mark up that is generated is the following
<link rel="stylesheet" href="/apps/mycomponents/MyFirstComponent/clientlib.css" type="text/css">
if you see the markup generated the link element is not closed properly, causing it to be xhtml uncompliant. I have even set the html5 doctype on the design mode.
Is this a know issue/behaviour.
Environment CQ 5.5
html5 is not an XHTML doctype. You need to pick an XHTML doctype for XHTML markup to be produced.