We offered a content-creation service to users and thought that the in-browser spell check would be good enough- turns out there's a LOT of people who prefer the old, Word 6 style of spellcheck where you enact a thing and then step through the errors one-by-one. We never actually built that but we STILL get emails asking for it, and we can tell from context that they're on modern browsers.
Would this even apply to something like TinyMCE? I thought (read:assumed) the in-browser spell checking only worked against textarea's and text inputs.
Most (if not all) WYSIWYG HTML editors like TinyMCE use a contenteditable container (e.g. div) and all current browsers offer spell check in the contenteditable element.
It actually is a TinyMCE problem. (Article says that Tiny used the non public URL and if it got back 500 error, just defaulted to "oh, no spelling errors then")
In TinyMCE you get both the browser-based spell checkers (via the right click context menu), and the spelling suggestions in TinyMCE (depending which spelling backend you're using: aspell, enchant, google spell, etc. http://www.tinymce.com/wiki.php/plugin:spellchecker)
We use TinyMCE and a .NET plugin that uses the GoogleSpell check service. It's dead right now and we are trying to find a solve for it.
I decided to use Yandex Speller since at the time I was researching this issue it was clear that Google Spell was an undocumented and unsupported feature. http://api.yandex.ru/speller/