sed on html pages

Hello,

i use static website generator and so i clean with tidy the page. But 
tidy make some mistakes and i think with sed is the way easy to fix it.

The empty lines i delete with sed -i '/^$/d' index.html, the div i delete
with sed -i 's|<[/]\?div[^>]*>||g' index.html. But i found not a way i can
break line and insert 4 steps of whitespace.

Before:

  <title>Silvio Siefke | Blog</title><!--[if lt IE 9]>
      <script src="/static/js/html5.js" type="text/javascript"></script>
    <![endif]-->  
  <link href="/static/css/style.css" rel="stylesheet" type="text/css">

After:

  <title>Silvio Siefke | Blog</title>

    <!--[if lt IE 9]>
      <script src="/static/js/html5.js" type="text/javascript"></script>
    <![endif]-->  

  <link href="/static/css/style.css" rel="stylesheet" type="text/css">

How can i relize?

Can i combined all step, delete empty line, delete div, id="ext", id="vid"
and this break line?


Thank you for help & Nice Day
Silvio