Except that the mobile browser does all that reflowing and resizing at the direction of CSS (its own fallback CSS in the absence of supplied applicable CSS).
Restricting CSS to any degree is in no way a solution here. What you really want is for people producing websites to bother understanding CSS and how to properly direct the design in a device-independent manner. Because CSS is fully capable of doing so at this point.
Mobile browsers do all kinds of special tricks that you need to manually disable. text-size-adjust is one of those properties that was added long after browsers artificially resized fonts to be larger than specified by CSS (and is still considered experimental from a standards point of view).
I wouldn't trust websites to apply my phone's font size the same way browsers do. Websites that do override the zoom factor often end up with huge fonts on my phone because I have the font size turned down (what's the point of a 6" slab of glass if you scale up the text so you get the same amount of information as on a 3" screen?).
Restricting CSS to any degree is in no way a solution here. What you really want is for people producing websites to bother understanding CSS and how to properly direct the design in a device-independent manner. Because CSS is fully capable of doing so at this point.