Hello! Welcome to Emacs!

Contrary to the other commenters, I would suggest starting with an out-of-the-box Emacs and only adding the things you need, as you need them.

As for your question, could you provide more detail about your expectations?

In the absence of it, I'll give you some generic responses:

• Shell-scripting: Emacs should support this out of the box
• Markdown: There is a popular package that provides a major mode markdown-mode: https://github.com/jrblevin/markdown-mode
• HTML+CSS: There is a major mode html-mode that is included by out of the box: https://www.gnu.org/software/emacs/manual/html_node/emacs/HTML-Mode.html
• For language support, the response will depend on your needs/expectations. If all you want is highlighting, it will probably work out of the box. For advanced features, you'll need to look at Eglot or something like lsp-mode