And you could argue that zooming in on an HTML bounding box would be obvious to an engineer developing web browsers.
In any case, if it's so obvious, where's the prior art? It's not as if web browsers haven't been around for nearly 20 years.
It seems to me that Apple has justification to defend wholesale copying of the myriad of little design decisions that they made that are synergistic. It's clear that they spent a huge amount of effort and cost doing this, and were able to succeed in a way that many had tried previously and failed.
The legal ways that one can defend such synergistic design work is limited. One common way, for better or worse, is to patent some of the individual elements to throw gum in the works of anyone trying to make derivatives of your work.
In practice, I'd take it a step further: Patents aren't about doing things that are hard. They're about doing things that are new... to the patent office.
But if the process isn't difficult, and we can assume that people individually have used zoom to zoom in on text, then the extent of the innovation is having it as a preset.