Gunning Fog
The Gunning Fog index is a reference to assess just how easy or difficult it is, to read a document.
It doesn't apply to font or styling, but more to the complexity of the written text. Complexity, for example, has three syllables which some people might find difficult to read.
It depends on context really. If you had a site which concerned itself with neurology, then you might not expect that a typical visitor, could not easily read the word, encephalitis.
But if the target market for something was in the age group of ten to thirteen years; then some considerations must surely be made.
Test an url document with this tool, perhaps.
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