bigtext.js

var BigText = {
    STARTING_PX_FONT_SIZE: 11,
    DEFAULT_MAX_FONT_SIZE_EM: 48,
    GLOBAL_STYLE_ID: 'bigtext-style',
    STYLE_ID: 'bigtext-id',
    LINE_CLASS_PREFIX: 'bigtext-line',
    LINE_FOCUS_CLASS: 'bigtext-focus',
    DEFAULT_CHILD_SELECTOR: '> div',
    childSelectors: {
        div: '> div',
        ol: '> li',
        ul: '> li'
    },
    counter: 0,
    init: function($head)
    {
        if(!$('#'+BigText.GLOBAL_STYLE_ID).length) {
            $head.append(BigText.generateStyleTag(BigText.GLOBAL_STYLE_ID, ['.bigtext { font-size: ' + BigText.STARTING_PX_FONT_SIZE + 'px; }']));
        }
    },
    getStyleId: function(elementId)
    {
        return BigText.STYLE_ID + '-' + elementId;
    },
    generateStyleTag: function(id, css)
    {
        return $('<style>' + css.join('\n') + '</style>').attr('id', id);
    },
    generateFontSizeCss: function(elementId, linesFontSizes, lineWordSpacings)
    {
        var css = [],
            styleId = BigText.getStyleId(elementId);

        for(var j=0, k=linesFontSizes.length; j<k; j++) {
            css.push('#' + elementId + ' .' + BigText.LINE_CLASS_PREFIX + j + ' { font-size: ' + linesFontSizes[j] + 'em;' + 
                (lineWordSpacings[j] ? ' word-spacing: ' + lineWordSpacings[j] + 'px;' : '') + ' }');
        }

        $('#' + styleId).remove();
        return BigText.generateStyleTag(styleId, css);
    },
    testLineDimensions: function($line, maxwidth, property, size, interval, units)
    {
        $line.css(property, size + units);

        if($line.width() >= maxwidth) {
            $line.css(property, '');

            return parseFloat((parseFloat(size) - interval).toFixed(2));
        }

        return false;
    }
};

$.fn.bigtext = function(options)
{
    var $headCache = $('head');
    BigText.init($headCache);

    options = $.extend({
                maxfontsize: BigText.DEFAULT_MAX_FONT_SIZE_EM,
                childSelector: ''
            }, options || {});

    return this.each(function()
    {
        var $t = $(this).addClass('bigtext'),
            id = $t.attr('id'),
            childSelector = options.childSelector ||
                        BigText.childSelectors[this.tagName.toLowerCase()] ||
                        BigText.DEFAULT_CHILD_SELECTOR,
            maxwidth = $t.width(),
            $c = $t.clone(true)
                        .addClass('bigtext-cloned')
                        .removeAttr('id')
                        .css({
                            'min-width': parseInt(maxwidth, 10),
                            width: 'auto',
                            position: 'absolute',
                            left: -9999,
                            top: -9999
                        }).appendTo(document.body);

        if(!id) {
            id = 'bigtext-id' + (BigText.counter++);
            $t.attr('id', id);
        }

        var styleId = BigText.getStyleId(id);
        $('#' + styleId).remove();

        // font-size isn't the only thing we can modify, we can also mess with:
        // word-spacing and letter-spacing.
        // Note: webkit does not respect subpixel letter-spacing or word-spacing,
        // nor does it respect hundredths of a font-size em.
        var fontSizes = [],
            wordSpacings = [];

        $c.find(childSelector).css({
            float: 'left',
            clear: 'left'
        }).each(function(lineNumber) {
            var $line = $(this),
                intervals = [16,8,4,2,1,.1,.01],
                fontMatch = 1,
                lineMax;

            for(var m=0, n=intervals.length; m<n; m++) {
                inner: for(var j=1, k=10; j<=k; j++) {
                    lineMax = BigText.testLineDimensions($line, maxwidth, 'font-size', fontMatch + j*intervals[m], intervals[m], 'em');

                    if(lineMax !== false) {
                        fontMatch = lineMax;
                        break inner;
                    }
                }

                if(fontMatch > options.maxfontsize) {
                    break;
                }
            }

            if(fontMatch > options.maxfontsize) {
                fontSizes.push(options.maxfontsize);
            } else {
                fontSizes.push(fontMatch);
            }
        }).each(function(lineNumber) {
            var $line = $(this),
                wordSpacing = 0,
                interval = 1,
                maxWordSpacing;

            // must re-use font-size, even though it was removed above.
            $line.css('font-size', fontSizes[lineNumber] + 'em');

            for(var m=0, n=10; m<n; m+=interval) {
                maxWordSpacing = BigText.testLineDimensions($line, maxwidth, 'word-spacing', m, interval, 'px');
                if(maxWordSpacing !== false) {
                    wordSpacing = maxWordSpacing;
                    break;
                }
            }

            $line.css('font-size', '');
            wordSpacings.push(wordSpacing);
        }).removeAttr('style');

        $headCache.append(BigText.generateFontSizeCss(id, fontSizes, wordSpacings));

        $c.remove();

        $t.find(childSelector).each(function(lineNumber)
        {
            $(this).each(function()
                {
                    // remove existing line classes.
                    this.className = this.className.replace(new RegExp('\\s*' + BigText.LINE_CLASS_PREFIX + '\\d+'), '');
                })
                .addClass(BigText.LINE_CLASS_PREFIX + lineNumber)
                [maxwidth / fontSizes[lineNumber] < 80 ? 'addClass' : 'removeClass'](BigText.LINE_FOCUS_CLASS);
        });
    });
};