for MDN

select_pre.js

/*
  make btn#selectPre
  click -> select pre-content
  
  remake - if webkit support mouseenter
*/
;(function(d){

if(!window.getSelection){
  return
}

var btn = d.createElement("button")
btn.id = "selectPre"
btn.textContent = "select"
btn.addEventListener("click", selectPre, false)

function selectPre(){
  window.getSelection().selectAllChildren(this.parentNode)
}


var pres = d.getElementsByTagName("pre")
for(var i=pres.length; i--;){
  pres[i].addEventListener("mouseover", addBtn, false)
  //pres[i].addEventListener("mouseout", remBtn, false) // if mouseleave
}


function addBtn(e){
  if(this === addBtn.ele) return // not to addBtn if already
  //console.log(e.target, e.currentTarget,e.relatedTarget)
  this.appendChild(btn)
  return addBtn.ele = this
}

/*
function remBtn(){
  btn.parentNode.removeChild(btn)
}
*/

})(document)

/*
CSS
pre{
  position: relative
}
#selectPre{
  position: absolute;
  top: 0;
  right: 0;
  border: none;
  padding: 2px 3px;
  font-family: consolas
}
*/