As explained in Topic 6, the DOM (Document Object Model) allows you to access and change any element within the document. This includes access to CSS styles and their properties. There are many things that can be done with this tool. The next few examples will show what can be done when an event occurs to the element.
Topic 7: Manipulating CSS Class Properties Using JavaScript
This is my teaching video. It starts at 3:46 and ends at 6:57.