Font scaling based on width of container

Question

I\'m having a hard time getting my head around font scaling.

I currently have a website with a body font-size of 100%. 100% of what though? This seems t

Answer 1

This worked for me:

I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"

The answer is a double proportion system:

{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }

Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value

function getMaxFontSizeApprox(el){
    var fontSize = 10;
    var p = el.parentNode;

    var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
    if(!parent_h)
        parent_h = 0;

    var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
    if(!parent_w)
        parent_w = 0;

    el.style.fontSize = fontSize + "px";

    var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
    if(!el_h)
        el_h = 0;

    var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
    if(!el_w)
        el_w = 0;

    // 0.5 is the error on the measure that JavaScript does
    // if the real measure had been 12.49 px => JavaScript would have said 12px
    // so we think about the worst case when could have, we add 0.5 to 
    // compensate the round error
    var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
    var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);

    fontSize = Math.floor(Math.min(fs1,fs2));
    el.style.fontSize = fontSize + "px";
    return fontSize;
}

NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.