XML Parsing Error: XML or text declaration not at start of entity

I'm not sure whether it's an XML specification requirement, but it appears that when rendering XML with Salesforce there cannot be any whitespace between the apex:page start tag and the first character.

The following code will work:

But the following code with visually pleasing indention will not:

If there are any unexpected spaces present after the apex:page start tag, you will likely encounter an error: XML Parsing Error: XML or text declaration not at start of entity