<title>CSS</title>

<style>
    body {
        font: 200%/1.45 charter;
    }
    ref::before {
        content: '\00A7';
        letter-spacing: .1em;
    }
</style>


<article>
 The seller can, under Business Law 
 <ref>
  1782
 </ref>, offer a full refund to buyers. 
</article>

<script>
    function ruleSelector(selector) {
        function uni(selector) {
            return selector.replace(/::/g, ':')
        }
        return Array.prototype.filter.call(Array.prototype.concat.apply([], Array.prototype.map.call(document.styleSheets, function(x) {
            return Array.prototype.slice.call(x.cssRules);
        })), function(x) {
            return uni(x.selectorText) === uni(selector);
        });
    }

    var toggle = false, 
        pseudo = ruleSelector("ref::before").slice(-1);

    document.querySelector("article").onclick = function() {
        pseudo.forEach(function(rule) {
            if (toggle = !toggle)
                rule.style.color = "red";
            else
                rule.style.color = "black";
        });
    }
</script>

这样应该可以了吧

为什么不能控制？

<style>
p:after{content:'我是后缀'}
</style>
<p>正文内容</p>

<script>
var css=function(t,s){
    s=document.createElement('style');
    s.innerText=t;
    document.body.appendChild(s);
};

document.onclick=function(){
    css('p:after{content:\'修改一下\'}');
};
</script>

首先after是伪元素，不是伪类。 其次是可以js改变它的样式的，操作思路是js更改style代码，而不是js选中元素再更改样式