As somebody already wrote "Starting from scratch is almost impossible to do alone". However you could set a first goal of doing some basic rendering of plain HTML, even text based. That should make you familiar with the hurdles of a rendering engine. Then apply some CSS. Even writing a browser for HTML 3.2 from scratch and no JavaScript should be an exercise to be proud of https://www.w3.org/TR/REC-html32 (1997). Then start looking at modern browsers using the links others provided. You should have the experience to understand all the moving parts by then.

This is similar to writing the very first web application without using any framework, maybe even interfacing directly with the web server. I remember decoding CGI arguments in C in 1994 before discovering Perl's CGI.pm. Then you understand the magic inside frameworks and don't get burned by surprises.