Currently the fullscreen element ready check is:

• element's namespace is the HTML namespace or element is an SVG svg or MathML math element.
• element is in a document.
• element's node document is allowed to use the feature indicated by attribute name allowfullscreen.
• element's node document's browsing context either has a browsing context container and the fullscreen element ready check returns true for element's node document's browsing context's browsing context container, or it has no browsing context container.

The final point is redundant, as it's guaranteed to be true by the previous two. The "allowed to use check" guarantees that if this isn't the top-level browsing context, then it's connected to it via iframes, which must be in documents, as removing an iframe discards its browsing context.

Will PR.