See https://twitter.com/lcamtuf/status/1420936558002151428

content.js

// Based on https://github.com/ericwbailey/millennials-to-snake-people.

function walkTextNodes(rootNode) {
    let walker = document.createTreeWalker(rootNode, NodeFilter.SHOW_TEXT,
        function(node) {
            if (node.textContent.length === 0) {
                return NodeFilter.FILTER_SKIP;
            }
            return NodeFilter.FILTER_ACCEPT
        },
        false);

    let node;
    while (node = walker.nextNode()) {
        handleText(node);
    }
}

function handleText(textNode) {
  textNode.nodeValue = replaceText(textNode.nodeValue);
}

function replaceText(value) {
    return value.replace(/\b(y|Y)our\b/g, "$1ou're");
}

function isForbiddenNode(node) {
    return node.isContentEditable ||
        (node.parentNode && node.parentNode.isContentEditable) ||
            (node.tagName && (node.tagName.toLowerCase() == "textarea" ||
                                 node.tagName.toLowerCase() == "input"));
}

function observerCallback(mutations) {
    let i, node;
    mutations.forEach(function(mutation) {
        for (i = 0; i < mutation.addedNodes.length; i++) {
            node = mutation.addedNodes[i];
            if (isForbiddenNode(node)) {
                continue;
            }
            if (node.nodeType === 3) {
                handleText(node);
            } else {
                walkTextNodes(node);
            }
        }
    });
}

(function (content) {
    let observerConfig = {
        characterData: true,
        childList: true,
        subtree: true,
    };

    walkTextNodes(content.body);
    content.title = replaceText(content.title);

    let bodyObserver = new MutationObserver(observerCallback);
    bodyObserver.observe(content.body, observerConfig);

    let title = content.getElementsByTagName('title')[0];
    if (title) {
        let titleObserver = new MutationObserver(observerCallback);
        titleObserver.observe(title, observerConfig);
    }
})(document);

manifest.json

{
    "manifest_version": 2,
    "name": "You're welcome!",
    "description": "You need it. Period.",
    "version": "0.1.0",
    "content_scripts": [
        {
            "matches": [
                "*://*/*"
            ],
            "all_frames": true,
            "js": [
                "content.js"
            ],
            "run_at": "document_end"
        }
    ]
}