Display a HTML file, where the font size can be changed

I need a simple feature added into a website where. When the reading mode button (similar to Safari Reading mode) in the toolbar is clicked - an overlay window is shown, - this window contains a html page for a URL which is being defined by a page number - in the top it should contain a - close button - (+) / (-) Button which increases the font size Here is the link: [login to view URL] There is a function, which gets the current page number when you go to another page. The HTML will be in the same folder like [login to view URL]