Making the buttons is easy, making the font change size not so easy.
What could be done:
You could modify the main XML file that contains the 'instructions' that make up the page content, the problem is changing the file on the fly. Unless the file is writable most likely on the local PC then it won't work.
You could have more than one version of the text in different sizes and just hide them, in which case the buttons are triggering simple show hide actions.
You could put in a little javascript that will replicate the user making the appropriate key presses to zoom the font in or out.
There you go three options already!
Making the buttons is easy, making the font change size not so easy.
What could be done:
You could modify the main XML file that contains the 'instructions' that make up the page content, the problem is changing the file on the fly. Unless the file is writable most likely on the local PC then it won't work.
You could have more than one version of the text in different sizes and just hide them, in which case the buttons are triggering simple show hide actions.
You could put in a little javascript that will replicate the user making the appropriate key presses to zoom the font in or out.
There you go three options already!