Easy to Get Started with
There are various neat libraries and tools you can use to easily build hybrid mobile apps or browser-based games, and they're often designed to be very beginner-friendly.
Difficult to Master
Scalability shouldn't be an issue for personal projects, but it is generally not as recommended for professional projects.
Currently you can only build simple browser games and simple hybrid apps using HTML5, and this is partially to the fact that many libraries rely on DOM, which usually have performance issues that limit the complexity or size of the game.
It is likely possible to build more complex browser based games in the future using Canvas, though this will not be beginner friendly.
As you step into the programming world, you'll soon understand how vital support is, as the developer community is all about giving and receiving help. The larger a community, the more likely you'd get help and the more people will be building useful tools to ease the process of development.
HTML is the 8th Most-used Tag on StackOverflow: HTML
StackOverflow is a programming Q&A site you will no doubt become intimate with as a coding beginner. HTML is the 8th most-used tag, which means there are a lot of questions with potential answers for your problem. In addition, CSS is the 12th most used tag.
Over 575k CSS Projects on GitHub
The more useful projects there are, the more likely someone has already built something you need and built it well, which will greatly speed up your development process. Over 350 CSS GitHub Projects have 500+ stars. In addition, CodePen is also a cool place to get inspiration or CSS Tips.
If you know HTML5 and CSS3 well, there's plenty of opportunity to find a job as a designer, especially since you'd have the advantage of being able to code what you've designed, reducing a lot of back and forth and also allowing you to have control over your own work.
As a skill, HTML5 and CSS3 is also useful for online marketers, since you won't have to always get company developers to help you fix something like the email template design if you can do it yourself.