Make visibility check optional
Make visibility check optional

There are use-cases where `document.documentElement` does not contain the element.

In my case I was contributing to a Chrome Extension, and `document.documentElement` contained the background page only, which made the plugin stop working.

There might be a smarter way to do this, i.e. finding out if the element is visible - maybe by specifying the parent in some way. But it is rather an edge case, so this solution should be sufficient.
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