Replace multiple <br>'s with only one <br>

CSS Solution

If you want to disable the effect of multiple <br> on the page, you can do it by CSS without using JavaScript:

br + br { display: none; }

• Check the jsFiddle demo.

However, this method is ideal when you are working with tags, something like this:

<div>Text</div><br /><br /><br />
<div>Text</div><br /><br /><br />
<div>Text</div><br /><br /><br />

In other cases, like this:

Hello World<br />   <br />
Hello World<br />   <br />
Hello World<br />   <br />

It will fail (as CSS passes text nodes). Instead, use a JavaScript solution.

---

JavaScript Solution

// It's better to wait for document ready instead of window.onload().
window.onload = function () {
    // Get all `br` tags, defined needed variables
    var br = document.getElementsByTagName('br'),
        l = br.length,
        i = 0,
        nextelem, elemname, include;
        
    // Loop through tags
    for (i; i < l - 1; i++) {
        // This flag indentify we should hide the next element or not
        include = false;
        
        // Getting next element
        nextelem = br[i].nextSibling;
        
        // Getting element name
        elemname = nextelem.nodeName.toLowerCase();
        
        // If element name is `br`, set the flag as true.
        if (elemname == 'br') {
            include = true;
        }
        
        // If element name is `#text`, we face text node
        else if (elemname == '#text') {
            // If text node is only white space, we must pass it.
            // This is because of something like this: `<br />   <br />`
            if (! nextelem.data.replace(/\s+/g, '').length) {
                nextelem = br[i+1];
                include = true;
            }
        }
        
        // If the element is flagged as true, hide it
        if (include) {
            nextelem.style.display = 'none';
        }
    }
};

• Check the jsFiddle demo.

What is the point of sending HTML, which is in a form that you don't want, to the client browser and making it run JavaScript code to clean it up? This looks like a bad design.

How about fixing all your static HTML, and HTML generation, so that these superfluous <br> elements do not occur in the first place?

If you use JavaScript to modify the document object, do so for dynamic effects that cannot be achieved in any other way.

Simpler:

var newText = oldText.replace(/(<br\s*\/?>){3,}/gi, '<br>');

This will allow optional tag terminator (/>) and also spaces before tag end (e.g. <br /> or <br >).