By default, SFDC pages (including Visualforce) are served with the DOCTYPE for HTML 4.01 Transitional (!!!) --
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
According the the VF documentation, we can serve VF pages as any doctype we want, including HTML5:
The HTML document type definition (DTD), or doctype, that describes the structure of the rendered page. If not specified, this value defaults to "html-4.01-transitional", which results in a doctype of . Possible values for this attribute include "html-4.01-strict", "xhtml-1.0-transitional", "xhtml-1.1-basic", and "html-5.0", among others. For more information about HTML doctype declarations, see the W3C specifications. -- Documentation for apex:page docType attribute
I'd love to develop my Visualforce in HTML5, but we're using a lot of standard Salesforce styling and formatting. Does the SFDC styling perform well under HTML5? Are there bugs, errors, or strange behaviors you've noticed?