That line would make your page HTML 4.01 strict. This could be a problem if you have a very picky browser, because Ext uses stuff like iframes and form targets.

I recommend using Ext either in quirks mode (no doctype) or with HTML 4.01 transitional:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "">
Also, the array-grid example creates invalid HTML (some of the company names contain &-signs that should have been escaped by a render. Try:
  id        : 'company',
  header    : 'Company', 
  width     : 160, 
  sortable  : true, 
  dataIndex : 'company',
  renderer  : 'htmlEncode'