HTML and CSS are languages that determine how a website looks and are quite different from the rest:
You know that famous saying "Once you learn one language, it's easy to learn others"? That saying is true, but it doesn't apply to these two; knowing HTML vs CSS does not prepare you for other languages.
If you are going to start right away with a mobile app, which language should you start with?
With Python, where no mobile apps were written. As absurd as it sounds, it's one of the best languages for beginners thanks to its intuitive syntax, and it's really easy to jump into other languages afterwards. Then you'll probably switch to Swift, where iOS apps are written, then Kotlin for Android, and your Python foundation will keep all those transitions smooth. Plus, under the Django framework, you can build any web application you can think of with Python.
What about Flutter?
Flutter is a framework that uses the Dart language and lets you simultaneously develop apps for Android, iOS, Windows, Mac, Linux and the web with a single code. In other words, you don't have to learn a different language and write separate code for each platform, and you don't struggle to create the same design over and over on different softwares. Sounds like a dream, doesn't it?
Maybe so — I'm not in a position to argue about it because I don't have experience myself. For me, due to the fact that top tech companies mostly prefer native applications (written and run separately for each platform) and to the flexibility of native, cross-platform is not on my radar yet. Moreover, there is the Apple side of things; Flutter is a SDK (software development kit) created by Google and them dominating the industry means Apple will also be giving up its own language Swift (and, in part, its software XCode). Hmmm...
Since the barrier to entry is low in terms of learning (easier than all the languages above) and required hardware (you don't need a Mac, you can run it on Windows), Flutter freelancers are very common, especially in developing countries. However, since most corporations still distance themselves from this platform, Flutter developers take longer to find a job than native developers.
Anyhow, you should know that Flutter is becoming more popular day by day, drastically shortening development time, and some titans including AliBaba, BMW and eBay develop their apps on Flutter.