Symptoms
Error "Your browser's compatibility mode is not supported" in SharePoint 2010 embedded K2 Forms.
Diagnoses
SharePoint 2010 forces IE 8 Document Mode via META tag in its master pages ().
Resolution
Change SharePoint's master page compatibility meta tag to IE=11 instead of IE=8:
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Note that this should be done specifically to pages that house K2 Forms as changing the document mode in SP 2010 can make some ribbon controls misbehave.
Additionally, you may be able to change the document mode via IIS, specifically for the K2 Runtime site, by modifying the 'X-UA-Compatible' HTTP Response Header on the K2 - Runtime site in IIS. The field should have values like the following: "IE=11,IE=10,IE=9,IE=8". Removing the lower versions would force Internet Explorer into a modern Document Mode for Runtime SmartForms, which may resolve this issue.
