Simple HTML-string beautifier (works in browser)

html-beautifier.js

function beautify( 
/* HTML-string */          html, 
/* new line chars */       nl = '\n', 
/* indent chars */         tab = '\t', 
/* disabled format tags */ disabled = /^(code|pre|em|strong|span|b|i|u|sup|sub|a)$/ig, 
/* without closed tags */  single = /^(area|base|br|col|command|embed|hr|img|input|keygen|link|menuitem|meta|param|source|track|wbr)$/gi
                ){
  return escape(  
    stringify(
      new DOMParser()           /* parse HTML */
      .parseFromString(html,'text/html')
      .documentElement          /* <root> element */
    )
  );

function stringify(el,depth=0){
  const name = el.tagName.toLowerCase();
  const indent = nl+''.padEnd(depth,'x').replace(/x/g,tab);
  const attrNames = el.getAttributeNames();
  const attrs = attrNames.map(n=>`${n}="${el.getAttribute(n)}"`).join(' ');
  let output = indent+(attrs.length ? `<${name} ${attrs}>` : `<${name}>`);
  if(el.childNodes.length){
    let prevDisabled=false;
    for(let node of el.childNodes){
      if(node.nodeType == 1){
        if(disabled.test(node.tagName)){
          output += node.outerHTML;
          prevDisabled = true;
        } else {
          output += stringify(node,depth+1);
          prevDisabled = false;
        }
      }
      else if([3,4,7,8].includes(node.nodeType)){
        if(!prevDisabled)
          output += indent+tab;
        output += node.nodeValue.split(/\n\s*/g).join(indent+tab);
        prevDisabled = false;
      }
    }
    output += indent+`</${name}>`;
  }
  else if(!single.test(name))
    output+=`</${name}>`;
  return output;
}
function escape(text){ return text
      .replace(/&/g, "&amp;")
      .replace(/</g, "&lt;")
      .replace(/>/g, "&gt;")
      .replace(/"/g, "&quot;");
 }
}

// test on current page
document.write('<pre style="tab-size:2;">'+beautify(document.documentElement.innerHTML)+'</pre>');