I've wondered why the spelling checker on a tech forum flags so many terms. Two from my previous post.
coupler's
directivity
As tooki said, the spell checker is part of your browser, not the web page.
I'm using Firefox. When it flags a word as misspelled that I know is spelled correctly, I right-click on the word and select "add to dictionary" and viola! the word is no longer flagged as misspelled. Sometimes I don't know for sure if the word is spelled correctly or not. In that case I highlight the word before right-clicking and choose "search the internet for [...]" Invariably, Duckduckgo will find the correct spelling. (I avoid all things Google whenever possible.). Then I fix it in my text, and/or add the word to Firefox's dictionary.