Simulates text-combine-upright: digits 4 (tate-chu-yoko) in Firefox

tateChuYoko.css

.num {
  text-combine-upright: all;
}

tateChuYokoify.js

// Wraps all numbers in a <span class="num">
function tateChuYokoify() {
  function walkText(node) {
    const regex = /([\d０-９]{1,4})/g;
    let matches = [];
    if (node.nodeType == 3) {
      while (match = regex.exec(node.data)) {
        match.index > 0 ? node.splitText(match.index) : node.splitText(match.index + match[0].length);
        const replacementNode = document.createElement("span");
        replacementNode.textContent = node.data;

        if (replacementNode.textContent.match(/^[\d０-９]{1,4}$/)) {
          replacementNode.className = "num";
        }

        node.replaceWith(replacementNode);
      }
    }

    if (node.nodeType == 1 && node.nodeName != "SCRIPT") {
      for (var i = 0; i < node.childNodes.length; i += 1) {
        walkText(node.childNodes[i]);
      }
    }
  }
  walkText(document.body);
}
tateChuYokoify();