Switching CSS classes based on screen size

CSS newby here...

I'm looking at a responsive framework and imagining how I would accomplish different tasks.

Based on the size of the screen, they have classes added to the body tag such as:

.PhoneVisible, .DesktopVisible, etc...

They also have classes to make links into buttons :

.btn, small-button, med-button, large-button

I'm puzzled on how you would go about changing your CSS. I.E. something like:

    <a href="#" class="MyButtonOptions">XXXX</>

    .PhoneVisible .MyButtonOptions { btn small-button }
    .TabletVisible  .MyButtonOptions { btn  med-button }
    .DesktopVisible .MyButtonOptions { btn large-button }

Do you have to set the varying options individually?

i.e. .PhoneVisible .MyButtonOptions { height:30px; } ?

All advice appreciated!

Solution

Take a look at this https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries.

Another way is to attach the resize event some piece of "switch code".

Something like this: http://jsfiddle.net/s5dvb/

HTML

<div id="body" class="limit400">
    <h1>Hey :D</h1>
</div>

CSS

.limit400 h1 { font-size:10px; }
.limit1200 h1 { font-size:50px; }

$(window).on('resize', function() {
    if($(window).height() > 400) {
        $('#body').addClass('limit1200');
        $('#body').removeClass('limit400');
    }else{
        $('#body').addClass('limit400');
        $('#body').removeClass('limit1200');
    }
})

About the frameworks, try http://purecss.io/ or http://getbootstrap.com/

Hope it helps.