JSFiddle - React, Tailwind, and code Playground

HTML

просто так

<p class="quote">
    цитата
    
    <br>Quote 
<BR  />хелло <Br>привет
</p>
    
    <p class="quoted">
        hello
    </p>

CSS

.quote { color: green; }

JavaScript

(function () {
    var observer = new MutationObserver(function(mutations) {
        mutations.forEach(function (mutation) {
            var i, node;
            
            if (mutation.addedNodes) {
                for (i = 0; i < mutation.addedNodes.length; i++) {
                    node = mutation.addedNodes[i];

                    if (node.classList && node.classList.contains("quote")) {
                        node.innerHTML = '&gt; ' + node.innerHTML.replace(/(<br[^>]*>)/gi, '$1&gt; ');
                    }
                }
            }
        });
    });

    observer.observe(document, {
        childList: true,
        subtree:   true
    });
}());