1) Add a Breadcrumb portlet to a page
2) ++Hit CTRL+U to view page source
Result: More than one H1 can be found under one section.
WCM SME's comment:
I reviewed the HTML page attached and there are four h1's:
nav > h1
section#portlet_com_liferay_site_navigation_menu_web_portlet_SiteNavigationMenuPortlet > ... > h1
section#content > h1
portlet_com_liferay_site_navigation_breadcrumb_web_portlet_SiteNavigationBreadcrumbPortlet_INSTANCE_VwVzewdai7aX > ... > h1 > ... > h2 > ... > h1
The last h1 is incorrect. Breadcrumb portlet is setting h1 tag regardless of the place where the portlet is rendered or how is rendered (embedded or not). I think that this might be explained by the fact that in the past Breadcrumb portlet would not render h1's because h1's were sorrounding the Breadcrumb markup but after a refactoring for 7.0 it started to render them inside causing this scenario when portlet is dragged to the page and not embedded in a theme.
So answering your question, this is not SEO-friendly or W3C-compliant (outline is not honored) at all, and it should be fixed.