Question

I have many elements with the same class on a web page. I would like to highlight all these elements when I hover one of them. How can I do that in CSS?

Right now, I have this CSS:

p.un:hover { background-color:yellow;}

And my HTML:

<div class="book">
  <div class="page left">
    <p class="un">Karen…</p>
  </div>
   <div class="page right">
     <p class="un">Karen ne se retourne pas. Mme Tilford reste là, apparemment confuse et abattue.</p>
   </div>

Answer 1

The best you can do using pure CSS is this:

.classname:hover ~ .classname {
    background-color: yellow;
}

But this only highlights all siblings with a class classname after the hovered element.

You have to use JavaScript to highlight all elements;

var elms = document.getElementsByClassName("classname");
var n = elms.length;
function changeColor(color) {
    for(var i = 0; i < n; i ++) {
        elms[i].style.backgroundColor = color;
    }
}
for(var i = 0; i < n; i ++) {
    elms[i].onmouseover = function() {
        changeColor("yellow");
    };
    elms[i].onmouseout = function() {
        changeColor("white");
    };
}

If you have multiple classes and want to generalize this, use something like this:

var classes = ["one", "two", "three"]; //list of your classes
var elms = {};
var n = {}, nclasses = classes.length;
function changeColor(classname, color) {
    var curN = n[classname];
    for(var i = 0; i < curN; i ++) {
        elms[classname][i].style.backgroundColor = color;
    }
}
for(var k = 0; k < nclasses; k ++) {
    var curClass = classes[k];
    elms[curClass] = document.getElementsByClassName(curClass);
    n[curClass] = elms[curClass].length;
    var curN = n[curClass];
    for(var i = 0; i < curN; i ++) {
        elms[curClass][i].onmouseover = function() {
            changeColor(this.className, "yellow");
        };
        elms[curClass][i].onmouseout = function() {
            changeColor(this.className, "white");
        };
    }
};