less gradient-vertical 方法的实现

// Vertical gradient using CSS where possible, and base64-encoded SVG for IE9 (enables use of this in combination with border-radius)
// Based on this by Phil Brown: http://blog.philipbrown.id.au/2012/09/base64-encoded-svg-gradient-backgrounds-in-less/
// Also based on a mixin from Twitter Bootstrap: https://github.com/twitter/bootstrap

.gradient-vertical(@startColor, @endColor) {

    // IE9 prep
    @dataPrefix: ~"url(data:image/svg+xml;base64,";
    @dataSuffix: ~")";
    @dataContent: ~'<svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" viewBox="0 0 1 1" preserveAspectRatio="none"><linearGradient id="g743" gradientUnits="userSpaceOnUse" x1="0%" y1="0%" x2="0%" y2="100%"><stop stop-color="@{startColor}" offset="0"/><stop stop-color="@{endColor}" offset="1"/></linearGradient><rect x="0" y="0" width="1" height="1" fill="url(#g743)"/></svg>';
    @b64Out: ~`(function(a,b,c){function e(a){a=a.replace(/\r\n/g,'\n');var b='';for(var c=0;c<a.length;c++){var d=a.charCodeAt(c);if(d<128){b+=String.fromCharCode(d)}else if(d>127&&d<2048){b+=String.fromCharCode(d>>6|192);b+=String.fromCharCode(d&63|128)}else{b+=String.fromCharCode(d>>12|224);b+=String.fromCharCode(d>>6&63|128);b+=String.fromCharCode(d&63|128)}}return b}function f(a){var b='';var c,f,g,h,i,j,l;var m=0;a=e(a);while(m<a.length){c=a.charCodeAt(m++);f=a.charCodeAt(m++);g=a.charCodeAt(m++);h=c>>2;i=(c&3)<<4|f>>4;j=(f&15)<<2|g>>6;l=g&63;if(isNaN(f)){j=l=64}else if(isNaN(g)){l=64}b=b+d.charAt(h)+d.charAt(i)+d.charAt(j)+d.charAt(l)}return b}var d='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';return a+f(b)+c})('@{dataPrefix}','@{dataContent}','@{dataSuffix}')`;

    background-color: mix(@startColor, @endColor, 60%); // Base solid colour fallback
    background-image: ~"@{b64Out}"; // IE9
    background-image: -moz-linear-gradient(top, @startColor, @endColor); // FF 3.6+
    background-image: -ms-linear-gradient(top, @startColor, @endColor); // IE10
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(@startColor), to(@endColor)); // Safari 4+, Chrome 2+
    background-image: -webkit-linear-gradient(top, @startColor, @endColor); // Safari 5.1+, Chrome 10+
    background-image: -o-linear-gradient(top, @startColor, @endColor); // Opera 11.10
    background-image: linear-gradient(top, @startColor, @endColor); // The standard
    background-repeat: repeat-x;

}