A red underline can be helpful, but its absence does not prove that a word is correct. Spellchecking depends on the editing context, browser support, settings, and language resources.
The HTML spellcheck attribute gives the browser a hint about checking editable content. MDN notes that it does not force the browser to perform a check in every situation.
Separate the request from the result
In a local practice page, place a deliberately misspelled ordinary word in an editable text area. Test the setting, then verify which language the browser is using. A product name or specialised term may behave differently from a common dictionary word.
For sensitive forms, also understand whether the browser’s enhanced checking sends text to a service. An HTML attribute alone is not a complete privacy policy or control over every user-installed tool.
Form authors should avoid treating underlining as their only feedback. Required fields, accepted formats, and server validation have separate jobs.
Readers can use spellchecking as one review pass, then examine names, dates, and meaning themselves. A correctly spelled word can still be the wrong word for the sentence, and a browser cannot settle the underlying factual claim.
Editorial illustration from this site’s image library; not documentary evidence of the example or object discussed.