If you want to control the viewport of a mobile browser, you're likely already using the so-called meta viewport. If you want to update the viewport—or perhaps set its value depending on some loadtime condition—there are (at least?) two ways to do this:
1) Append a new
meta element to the page.
2) Update the existing
Here's a sweet test page with some sweet buttons you can click on your mobile device to see both methods: [really sweet test page].
(Naturally I just hacked up a page from Andreas Boven's cool Understanding Viewport.)
If you scroogle "dynamically update meta viewport", you'll likely end up at PPK's blog post on the topic—but that was written back in 2011 when Beatlemania and Communism were still a thing.
Here's an updated account:
Appending a new
meta element works in every iOS and Android browser I could get my hands on (unfortunately I don't have any Windows Phone devices—but my friend reported that it didn't update on his Windows Phone).
meta @content works in everything I tried except Mobile Firefox browsers and Opera Mini (and IE Mobile 10-ish?). I don't expect Opera Mini to ever be updated, but you never know—Communism did eventually fail.
I filed bugs for Firefox for Android and Firefox OS because I think they should behave like Android and iOS browsers for compatibility reasons. (I also think it's more intuitive to change an attribute, rather than add a new element to the document.)