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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:
Also, the array-grid example creates invalid HTML (some of the company names contain &-signs that should have been escaped by a render. Try:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
id : 'company',
header : 'Company',
width : 160,
sortable : true,
dataIndex : 'company',
renderer : 'htmlEncode'