Try MDN Docs, DevDocs or WWW3Schools

/* DOM Manipulation

* If you want to update / add single style in DOM Element style attribute you can use this function:

* inject javascript after page reloads.

*/

​

function setCssStyle(el, style, value) {

  var result = el.style.cssText.match(new RegExp("(?:[;\\s]|^)(" +

      style.replace("-", "\\-") + "\\s*:(.*?)(;|$))")),

    idx;

  if (result) {

    idx = result.index + result[0].indexOf(result[1]);

    el.style.cssText = el.style.cssText.substring(0, idx) +

      style + ": " + value + ";" +

      el.style.cssText.substring(idx + result[1].length);

  } else {

    el.style.cssText += " " + style + ": " + value + ";";

  }

}

var element = document.getElementById("wallIframe");

setCssStyle(element, "display","none !important");