Without the wrapper, Chrome (and Opera) would use two pages for one page.

The tracemonkey report results in blank pages between the odd pages. In colored documents, it becomes viaully obvious that the bottom of theprevious page is rendered at the second page.

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I've added a brief note in the commit message https://github.com/Rob--W/pdf.js/commit/b1a9e8a1ae0eed7eee34a2be68111841a20ab576:

>  **Wrap printed &lt;canvas&gt; in &lt;div&gt; for Chrome**

> Otherwise one page (canvas) will consume two (printed) pages in Chrome.
This method has no side-effects in Firefox.