LanguageTool is a free grammar checker and paraphraser for English, Spanish, and 20 other languages. Instantly check your text for grammar and style mistakes.
Their extension isn't open source anymore, see here, so I don't recommend it personally, especially with how sensitive the data it collects is, its basically a keylogger, so trust is super important imo.
While I appreciate the sentiment, isn't that effectively the same as "if the restaurant won't give you the complete recipe for their pizza sauce, how do you know they're not trying to poison you?"