Learn Web/Browser APIs
Image source: http://www.evolutionoftheweb.com/
The BOM (Browser Object Model) and the DOM (Document Object Model) are not the only browser APIs that are made available on the web platform inside of browsers. Everything that is not specifically the DOM or BOM, but an interface for programming the browser could be considered a web or browser API (tragically in the past some of these APIs have been called HTML5 APIs which confuses their own specifics/standardize with the actual HTML5 specification specifying the HTML5 markup language). Note that web or browser APIs do include device APIs (e.g., Navigator.getBattery()) that are available through the browser on tablet and phones devices.
You should be aware of and learn, where appropriate, web/browser APIs. A good tool to use to familiarize oneself with all of these APIs would be to investigate the HTML5test.com results for the 5 most current browsers.
MDN has a great deal of information about web/browser APIs.
- MDN Web API Reference
- MDN Web APIs Interface Reference - All Interfaces, Arranged Alphabetically
- MDN WebAPI - Lists Device Access APIs and Other APIs Useful for Applications
Keep in mind that not every API is specified by the W3C or WHATWG.
In addition to MDN, you might find the following resources helpful for learning about all the web/browser API's: