Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Returns the MIME type that the document is being rendered as. This may come from HTTP headers or other sources of MIME information, and might be affected by automatic type conversions performed by either the browser or extensions.
Syntax
contentType = document.contentType;
contentType is a read-only property.
Notes
The property is not affected by META tags.
Specifications
Non-standard, only supported by Gecko. To be used in browser chrome code (i.e. Extensions and XUL applications).