Hi I know you said in CSS but as everybody told you, you can't, this is a javascript solution, just my 2 cents.
best...
JSFiddle
css
span.highlight{
background:#F60;
padding:5px;
display:inline-block;
color:#FFF;
}
p{
font-family:Verdana;
}
html
<p>
Let's go Zapata let's do it for the revolution, Zapatistas!!!
</p>
javascript
jQuery.fn.highlight = function (str, className) {
var regex = new RegExp(str, "gi");
return this.each(function () {
this.innerHTML = this.innerHTML.replace(regex, function(matched) {return "<span class="" + className + "">" + matched + "</span>";});
});
};
$("p").highlight("Z","highlight");
Result
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…