Is it possible to lock the view port's orientation to portrait on a mobile device?
I Googled it but could not find out exactly how to do it.
As my experience, it cannot be done as you are access the website from the browser. Which should have the lock orientation capability is the browser itself - e.g Safari, Chrome. Your HTML CSS code will not be feasible to control it.
For example when you are building a hybrid mobile app - meaning an application with html css and js then convert it to mobile app with wrapper as a web view inside. Then you are capable to lock the screen orientation. There are some configuration need to be done and later on these will be converted to Objective C or Java.