First suggestion: use body instead of html for your formatting. Like:
<style type='text/css'>
html{
font-size: 300%;
}
</style>
Even so, you should just delete that at the beginning so the font size will be variable.
if($_SESSION['FontSize']=='')
{
?>
<style type='text/css'>
body {
font-size: 100%;
}
</style>
<?php
}
The browser doesn't know that the <style> is in an if statement. Any thing that is not in the <?php ?> tags is ignored by PHP, but even if you didn't drop out of PHP.
I would suggest:
if(!isset($_SESSION['FontSize']))
{
echo '<style type='text/css'>
body {
font-size: 100%;
}
</style>';
}
That will output your style to the browser, so it's interpreted only when FontSize is blank.
I also added an isset so that it checks if the variable is not set, rather than if it is blank.
There may be a few other errors, I'm not sure; I'm no expert.