Website accessibility has always been important, but nowadays, when we have clear standards and regulations from governments in most countries, it's become even more crucial to support those standards and make our projects as accessible as they can be.
The W3C recommendation provides 3 level of conformance: A, AA and AAA. To be at the AA level, among other requirements, we have to provide a way to increase the site's font size:
1.4.4 Resize text: Except for captions and images of text, text can be resized without assistive technology up to 200 percent without loss of content or functionality. (Level AA)
Let's look at solutions for this and try to find the best one we can.
Update(09/25/17): It turns out that WCAG doesn't require a text resizing custom solution in addition to what user-agents provide. We just have to make sure a website looks OK as a result of resizing. Yet if we still want to scale elements for any reason, below is a thorough analysis of different methods for doing that.