How to Dynamically Scale Text Size Based on Browser Width

Font scaling based on width of container

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.

If the container is the body, what you are looking for is Viewport-percentage lengths:

The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.

The values are:

vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)

1 v* is equal to 1% of the initial containing block.

Using it looks like this:

p {
    font-size: 4vw;
}

As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.

These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.

Browser support is pretty good, but you'll likely need a fallback, such as:

p {
    font-size: 16px;
    font-size: 4vw;
}

Check out the support statistics: http://caniuse.com/#feat=viewport-units.

Also, check out CSS-Tricks for a broader look: Viewport Sized Typography

Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography

And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu

Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

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

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

Pure CSS to make font-size responsive based on dynamic amount of characters

Note: This solution changes based on viewport size and not the amount of content

I just found out that this is possible using VW units. They're the units associated with setting the viewport width. There are some drawbacks, such as lack of legacy browser support, but this is definitely something to seriously consider using. Plus you can still provide fallbacks for older browsers like so:

p {
    font-size: 30px;
    font-size: 3.5vw;
}

http://css-tricks.com/viewport-sized-typography/
and
https://medium.com/design-ux/66bddb327bb1

Increase font-size to full width of screen

in gumroad its not a text its an image
but if you want to control your font size you can use
font-size: clamp(min size , 2vw, max size)
for example font-size: clamp(14px, 2vw, 20px)
you can read more here https://developer.mozilla.org/en-US/docs/Web/CSS/clamp()

Dynamically Resize Text Based on Container Width

window.resize is the correct event but it doesn't fire on page-load. You can however just add .trigger('resize') to your code to make it fire on page-load:

$(window).bind('resize', function(){
    var containerSize  = $('.container').width(),
        textPercentage = 0.17391304347826086956521739130435, /* 40/230 */
        textRatio      = containerSize * textPercentage,
        textEms        = textRatio / 14;

    $('.container h3').css(fontSize, textEms+"em");
}).trigger('resize');

You are going to want to run this code after document.ready to make sure the width you are getting for the container is correct. You could also place this code at the bottom of your HTML document (which you should do with or without the document.ready event handler) which will make sure the elements are available when you run this code:

//wait for `document.ready` to fire
$(function () {

    //cache the .container and H3 elements
    var $container = $('.container'),
        $h3        = $container.find('h3');

    //bind event handler to `window.resize`
    $(window).bind('resize', function(){

        //get the width of the container
        var containerSize  = $container.width(),
            textPercentage = 0.17391304347826086956521739130435, /* 40/230 */
            textRatio      = containerSize * textPercentage,
            textEms        = textRatio / 14;

        $h3.css('fontSize', textEms+"em");
    }).trigger('resize');
});

Notice that I cached the H3 element(s) so it/then don't have to be selected every resize event, because when you actually re-size your browser there are tons of these events that fire.