LingoTweaker by Johannes Wilm

LingoTweaker proofreads the text you type on the web — plain input fields,
contenteditable areas, and rich-text editors such as ProseMirror/TipTap, CKEditor 5,
TinyMCE, Quill, CodeMirror 6, Slate, Lexical and Draft.js — with the open
LingoTweaker engine compiled to WebAssembly.

Private by design. The engine and all language data run on your own machine
inside the extension. No text is sent anywhere: there are no analytics, no
remote API calls and no accounts. The extension makes zero network requests
while checking.

Features

• Grammar, spelling and style checks with suggestions, in 38 languages.
• Click an underline for a suggestion card: apply a fix, add a word to your
  personal dictionary, ignore a rule, or disable the extension per site.
• Pick the checking language per site, or switch quickly with a keyboard shortcut.
• Editor-aware corrections that keep bold, italic and links intact.
• Opt-in deep integrations for Google Docs, Microsoft Word online,
  Fidus Writer and Gmail (off by default; enable in settings, then reload the
  editor tab).