Is it possible to dynamically scale text size based on browser width?

Here is the project this is for: http://phlak.github.com/jColorClock/. As you can see, right now the text size is just set to a static size. I'd like the text to always be ~90% of the width of the window but to also scale the vertical size accordingly. Is there a relatively easy way of doing this?

Hell yeah!

Set your <body> font size when the window is resized with a little javascript. (I've used jQuery for convenience here:

$( document ).ready( function() {
            var $body = $('body'); //Cache this for performance

            var setBodyScale = function() {
                var scaleSource = $body.width(),
                    scaleFactor = 0.35,                     
                    maxScale = 600,
                    minScale = 30; //Tweak these values to taste

                var fontSize = scaleSource * scaleFactor; //Multiply the width of the body by the scaling factor:

                if (fontSize > maxScale) fontSize = maxScale;
                if (fontSize < minScale) fontSize = minScale; //Enforce the minimum and maximums

                $('body').css('font-size', fontSize + '%');
            }

            $(window).resize(function(){
                setBodyScale();
            });

            //Fire it when the page first loads:
            setBodyScale();
        });

Because your font size is set in em's (perfect) adjusting the percentage font-size of the body element acts as a universal 'text zoom'. This will scale any text set in em's - if you want to be more specific, you could set the percentage font-size on a <div> that surrounds just the elements you want to scale.

Here's a quick example: http://www.spookandpuff.com/examples/dynamicTextSize.html

New units were added in CSS3 that will allow you to do this. Sitepoint has a good overview. You definitely want to provide a fallback for older browsers, but this is by far the simplest solution:

font-size: 35vmin;

Another option for when you don't need as much precision (say, a couple sizes for different devices) is to use media queries.

Same as Beejamin's excellent answer, with a couple tweaks.

The math was adjusted so that you can set the "default width" at which no scaling will occur. This makes it easier to design to a given width with exact font-sizes.
The font-size is now set on the html element freeing up the body element to hold a font-size in the css.

$(function() {

  // At this width, no scaling occurs. Above/below will scale appropriately.
  var defaultWidth = 1280;

  // This controls how fast the font-size scales. If 1, will scale at the same 
  // rate as the window (i.e. when the window is 50% of the default width, the 
  // font-size will be scaled 50%). If I want the font to not shrink as rapidly 
  // when the page gets smaller, I can set this to a smaller number (e.g. at 0.5,
  // when the window is 50% of default width, the font-size will be scaled 75%).
  var scaleFactor = 0.5;

  // choose a maximum and minimum scale factor (e.g. 4 is 400% and 0.5 is 50%)
  var maxScale = 4;
  var minScale = 0.5;

  var $html = $("html");

  var setHtmlScale = function() {

    var scale = 1 + scaleFactor * ($html.width() - defaultWidth) / defaultWidth;
    if (scale > maxScale) {
      scale = maxScale;
    }
    else if (scale < minScale) {
      scale = minScale;
    }
    $html.css('font-size', scale * 100 + '%');
  };

  $(window).resize(function() {
    setHtmlScale();
  });

  setHtmlScale();
});

Is it possible to dynamically scale text size based on browser width?

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