One of the first web browsers in the 90s was written by one dude in four days, with the whole GUI. A browser doesn't necessarily mean CSS and JS.
Sometime in the past couple months, there was a link on HN to a project also by one dude, who's implementing a CPU on breadboards, with wires. (Can probably be found by the score filter from the ‘undocumented features’ of HN: the post got >2000 points.)
Also, implementing browsers (or operating systems, or cpus) isn't about needing to write amounts of code that would take a long time write - it's about whether you know the subject area and do you already have a plan/architecture in your head that you are going to create.
Sometime in the past couple months, there was a link on HN to a project also by one dude, who's implementing a CPU on breadboards, with wires. (Can probably be found by the score filter from the ‘undocumented features’ of HN: the post got >2000 points.)
Computers aren't necessarily gigantic corporate undertakings.