Too many devs are intimidated by the Web and Internet standard specifications. I agree that it is impossible to read them cover-to-cover (however, with most RFCs and the REST dissertation, you can and should actually do that). Still, when you need to understand a particular section, usually you will find there many interesting details and all of the advanced concepts derived from first principles.
I mean specifically:
- the Internet Standards (RFCs). Read some of them [1] or browse all of them [2]
- the HTML5 living standard [3]. Actually the previous HTML specifications are also interesting, easier for reading and mostly compatible [9]
- ECMAScript living standard [4] or the latest finalized spec [5]
- CSS specifications [6] (they are modular and build on top of each other)
- the original REST proposal formulated in Roy Fielding's dissertation "Architectural Styles and the Design of Network-based Software Architectures": "Chapter 5: Representational State Transfer (REST)" [7]. It is also reformulated in "Principled Design of the Modern Web Architecture" [8]
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[1] https://security.stackexchange.com/questions/116893/which-ar...
[2] https://www.rfc-editor.org/retrieve/
[3] https://html.spec.whatwg.org/multipage/
[5] https://262.ecma-international.org/11.0/
[6] https://www.w3.org/Style/CSS/read.en.html
[7] https://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm
[8] https://www.ics.uci.edu/~taylor/documents/2002-REST-TOIT.pdf