VA11y
VA11y is a unified accessibility testing tool. It provides checks for accessible names, image text alternatives, heading structure, semantic page structure, text-spacing, and grayscale.
Bookmarklets
To use a bookmarklet, save it as a bookmark by dragging the link to your bookmarks bar or by manually creating a new bookmark and pasting the JavaScript code into the URL field. Once saved, navigate to the webpage you want to use it on and click the bookmarklet from your bookmarks bar to execute the code.