Is programming in C99 relevant for systems?

What at the best resources to learn Modern (C99) portable C for a person with some computing science background and a I have used Pascal, python... but I don’t have any knowledge of some other scripting languages like JavaScript, hypertext like HTML, css.

Let me know what you think: I all ready went throughout Programming in C and I am know proceeding with “Notes on Data Structures and Programming Techniques (YaLe CPSC 223, Spring 2018) by Yaleby James Aspnes and Modern C by Jens Gustedt.

Could you advise on good courses in Advanced C or books that might enlighten me.