CSS animate circle border filling with color

Question

I'm new to learning about CSS animations. I have a circle with an arc going around it, but I want to make it leave a trail behind. The idea is I'd use this to visually show how many credits somebody has used e.g. 75 / 100 credits used, the circle border becomes 3/4 colored-in.

I have a fiddle to show the arc stopping at the top. What I need is a way to

1) Make it stop at different points depending on the number of credits (can I use the 0%, 50% etc. inside keyframes somehow, like adding a class via jQuery?)

2) Leave a trail behind, so it fills with color.

.circle {
/* code - pls see fiddle */

@keyframes animation {
0% {transform: rotate(0);}
50% {transform: rotate(180deg);}
100% {transform: rotate(360deg);}
}

Answer 1

There is a very easy to follow, informative and detailed tutorial on exactly how to achieve this (and more) by Anders Ingemann, which can be found here.

Its a fairly complex operation- so I'll simply distil the final stage from the tutorial here

Demo Fiddle

HTML

<div class="radial-progress">
    <div class="circle">
        <div class="mask full">
            <div class="fill"></div>
        </div>
        <div class="mask half">
            <div class="fill"></div>
            <div class="fill fix"></div>
        </div>
        <div class="shadow"></div>
    </div>
    <div class="inset"></div>
</div>

CSS/LESS

.radial-progress {
    @circle-size: 120px;
    @circle-background: #d6dadc;
    @circle-color: #97a71d;
    @inset-size: 90px;
    @inset-color: #fbfbfb;
    @transition-length: 1s;
    @shadow: 6px 6px 10px rgba(0, 0, 0, 0.2);
    margin: 50px;
    width: @circle-size;
    height: @circle-size;
    background-color: @circle-background;
    border-radius: 50%;
    .circle {
        .mask, .fill, .shadow {
            width: @circle-size;
            height: @circle-size;
            position: absolute;
            border-radius: 50%;
        }
        .shadow {
            box-shadow: @shadow inset;
        }
        .mask, .fill {
            -webkit-backface-visibility: hidden;
            transition: -webkit-transform @transition-length;
            transition: -ms-transform @transition-length;
            transition: transform @transition-length;
        }
        .mask {
            clip: rect(0px, @circle-size, @circle-size, @circle-size/2);
            .fill {
                clip: rect(0px, @circle-size/2, @circle-size, 0px);
                background-color: @circle-color;
            }
        }
    }
    .inset {
        width: @inset-size;
        height: @inset-size;
        position: absolute;
        margin-left: (@circle-size - @inset-size)/2;
        margin-top: (@circle-size - @inset-size)/2;
        background-color: @inset-color;
        border-radius: 50%;
        box-shadow: @shadow;
    }
}

Example jQuery (could be substituted with CSS)

$('head style[type="text/css"]').attr('type', 'text/less');
less.refreshStyles();
var transform_styles = ['-webkit-transform', '-ms-transform', 'transform'];
window.randomize = function () {
    var rotation = Math.floor(Math.random() * 180);
    var fill_rotation = rotation;
    var fix_rotation = rotation * 2;
    for (i in transform_styles) {
        $('.circle .fill, .circle .mask.full').css(transform_styles[i], 'rotate(' + fill_rotation + 'deg)');
        $('.circle .fill.fix').css(transform_styles[i], 'rotate(' + fix_rotation + 'deg)');
    }
}
setTimeout(window.randomize, 200);
$('.radial-progress').click(window.randomize);