MDN contains user-generated content written in HTML. Most contributors use CKEditor, the WYSIWYG interface, which allows them to write content without knowing HTML.
CKEditor ships with a plugin for SCAYT , which implements “spell check as you type”, giving a similar experience to rich text editing in Word and Google Docs. We’ve made this plugin available on MDN, but there are some concerns with its continued and expanded usage (such as bug 1318827 , "Enable spell check by default). These issues are:
* It presents advertisements to those who use the spellcheck feature, which can be disabled with a license .
I have only seen advertisements for the licensed SCAYT service. It is possible that other ads may be served in the future.
Due to these concerns, I can't support turning on spell check by default. I also think that we can't continue using SCAYT without resolving this license issue, which would require time and resources that are in short supply.
My recommendation is to disable SCAYT and replace it with browser-based spell check. This may require disabling the integrated context menu as well, which gives CKEditor-specific options when you right-click. I'm looking for feedback on this recommendation, especially from those who are regular users of spell check and / or the context menu. I suspect that very few MDN users use either feature, and switching to browser-based spellcheck would be an improvement for almost everyone.
I've written a Google doc  that goes into more technical detail about the issues with SCAYT and the options I considered.