Document.charset is a non-standard IDL attribute supported by all engines except Gecko, with a proposal to standardize it: https://www.w3.org/Bugs/Public/show_bug.cgi?id=27436 In order to simplify standardization, Document.charset has been made readonly, as a simple alias of Document.characterSet.
Docs: https://www.w3.org/Bugs/Public/show_bug.cgi?id=27436 https://developer.mozilla.org/en-US/docs/Web/API/Document/characterSet