Create feed previews using a stream filter Instead of replacing the feed document with an XHTML preview from a content script after it has already been rendered, create an XHTML preview using a stream filter before it is passed into the rendering engine and use an XSL style sheet to convert it to HTML. This has two advantages, firstly it results in an HTMLDocument with the full HTML DOM available and secondly it avoids rendering the document twice. Refactor the feed preview creation and split parsing and rendering into seperate modules.

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