block-nested-in-inline.js

(function(){
  const inlineTags = new Set(['span','a','em','strong','b','i','small','sub','sup','mark','q','time','button']);
  const blockTags  = new Set(['div','p','ul','ol','li','section','article','header','footer','nav','figure','table']);
  function walk(node){
    for(let child of node.children){
      const p = node.tagName.toLowerCase();
      const c = child.tagName.toLowerCase();
      if(inlineTags.has(p) && blockTags.has(c)) {
        console.warn(`Invalid nesting: <${p}> → <${c}>`, node, child);
      }
      walk(child);
    }
  }
  walk(document.body);
})();

block-nested-in-inline.js bookmarklet

javascript:(function(){const inline=new Set(['span','a','em','strong','b','i','small','sub','sup','mark','q','time','button']),block=new Set(['div','p','ul','ol','li','section','article','header','footer','nav','figure','table']);(function w(n){for(let c of n.children){if(inline.has(n.tagName.toLowerCase())&&block.has(c.tagName.toLowerCase()))console.warn('Invalid ⟨'+n.tagName.toLowerCase()+'⟩→⟨'+c.tagName.toLowerCase()+'⟩',n,c);w(c);} })(document.body);})();