You need to use css and javascript together. For instance in you want to change font size:
<script>
document.getelementById('element_to_change').style.font-size= 24;
</script>

<div id="element_to_change">change this font</div>

http:www.customdesignsolutions.net

First you need to apple a font-size percentage to the body tag within the HTML:

Body
{
    font-size: 78.9%;
}

Then for every other element specify the font-size you're going to use (i.e h1, h2, and so on):

H1
{
    font-size: 1.8em;
}

p
{
    font-size: 1.2em;
}

Then the visible content:

<div id="sizeUp">bigger text</div>
<div id="normal">normal text</div>
<div id="sizeDown">smaller text</div>