HTML5 has emerged over Flash (the .swf or Adobe Flash file format used for multimedia, vector graphics & more) offering eLearning the flexibility of an open web standard (no plug-in required) to create courses (and webpages) with dynamic interactions, media, graphics, animation and more. HTML5 is an open web standard enabling multimedia content developed with it to be integrated into the website source code. HTML5 for eLearning makes a course responsive and compatible with different devices, browsers, OS, and online platforms, thus course deployment can be easier. The higher the course designer's comfort zone with HTML5 the more features or troubleshooting can be done when designing and deploying courses or client websites online. .
In CSS it's the selectors that target specific HTML elements and allow us to "style" them. This keeps the files separate and keeps the actual "design" happening in a separate "document". In this case, main.css. The selectors are pretty amazing in the level of specificity they offer in styling a page. The class selectors allow the creation and styling of as many detailed sections as one wants to create on an HTML page. These are well supported in all browser types. There can be multiple classes on an HTML element.