I found this web page which describes in detail a browser plugin that exposes Wacom tablet features such as pressure sensitivity in JavaScript. Unfortunately, the download link is dead. Does this product exist?
Pressure sensitivity is now exposed by the browser's Pointer Events API for all supported tablets (not just Wacom). Pointer events such as onpointermove will contain a "pressure" field that ranges between 0.0 and 1.0.
Pointer Events are supported by Chrome, Firefox, Internet Explorer (10+), Microsoft Edge and Safari (13+) on macOS and Windows 8+ (Windows 7 doesn't provide pressure information). See: