Implementation notes

HTML checker

Under the hood, most of the HTML checks are powered by axe, an engine for automated testing of HTML from Deque Systems.

Headless browser

During the evaluation of an EPUB, Ace loads each HTML content document in a “headless” web browser window (i.e. invisible from the point of view of the user of the command line tool, but nonetheless using a real rendering engine). Ace was originally based on Puppeteer but it is now using Electron by default. Electron and Puppeteer both use Chromium which is the open source component that powers various web browsers (including Google Chrome and Microsoft Edge). Both Ace (command line) and Ace App (Graphical User Interface) now rely on this Electron runtime to perform the accessibility checks, but the legacy Puppeteer method remains available via the command ace-puppeteer (similarly, the default Electron variant can also be explicitly invoked with ace-electron instead of ace).