Grammarly

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Do not build this. Working inside every text box is the product, and it is a permanent maintenance job.

A browser extension that behaves correctly in every editor on the web — rich text, code editors, contenteditable, shadow DOM, an email client that rewrites the DOM under you — is endless work, and the suggestions themselves are trained on a corpus you do not have.

WHAT TO DO INSTEAD
- Keep it if you write in other people's text boxes all day
- **And consider what it sees.** An extension with permission to read every text field is reading your email drafts, your messages and anything you type into an internal tool. That is a real consideration for work, and some employers forbid it for exactly that reason

WHAT YOU CAN BUILD, AND SHOULD

**Your own style checker**, which is better than any general one for your own writing:
- Run the open LanguageTool server in a container for grammar and spelling. Twenty languages of real linguistic work, free, and better than anything you would write
- On top of it, a term list: prefer this word, never that one, with a reason. Your product's name capitalised exactly one way, the hedges you overuse, the phrases you have decided against
- Rules as a file in a repository, reviewed like code. A style guide nobody can diff is one nobody follows
- Run it in CI on every pull request for documentation, and through an editor integration via the language server protocol, which reaches most editors at once
- Record which rules fire and get ignored. Delete any rule ignored twice — a rule nobody agrees with trains people to ignore the tool

AND THE MEASUREMENTS THAT NEED NO MODEL
Sentence length distribution rather than the average, repeated sentence openers, repeated phrases across a document, and passive constructions counted and located. All arithmetic, all more useful than a grammar correction, and all yours.

THE ONE-LINE VERSION
Keep it for other people's text boxes if you trust it there. Build the term list and run it in CI, because that is the part that actually improves your writing.