LaTeX isn't intended to take layout control away from the author so much as it is intended to automatically produce a good-enough layout allowing a single author to produce a very large document without employing a designer.
HTML by contrast explicitly does remove control over layout from the author and place it in the hands of the user (and their chosen user agent).
Both languages have mechanisms to (somewhat) separate the content from the formatting rules.
LaTeX would rather produce a bad document if it cannot produce a good one. Example: overfull hbox.
A designer is still required who creates the documentclass, it is just that LaTeX comes with some predefined ones intended for scientific publishing.
HTML+CSS require pixel-perfect rendering. Example: ACID2 test.
While it might have been the idea of plain HTML at some point (<em> instead of <i>), control has never been taken away from the author thanks to CSS.
You're badly mischaracterizing tests like ACID2. The test definition includes a long list of things that invalidate the test, including things like changing the zoom level. So it's wrong to construe that test as requiring pixel-perfect rendering when it explicitly doesn't cover exactly the kind of user agent controls I'm talking about.
Your comments about LaTeX do not seem to contradict anything I said.
HTML by contrast explicitly does remove control over layout from the author and place it in the hands of the user (and their chosen user agent).
Both languages have mechanisms to (somewhat) separate the content from the formatting rules.