Is there a free tool for rendering a webpage to a PNG with a transparent background?

Say I have a very simple html page, containing a single line of text. I'm looking for a tool that would take that line of text, and superimpose it on a transparent P开发者_StackOverflow社区NG background. That logic would be applied to ALL elements on the page: if no background is specified, and the background of parent elements doesn't leak through, then a background should render as transparent.

As in transparent-pixel-in-image tranparent.

Is there a tool for this?

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There are various command line tools for this sort of thing, most of which depend on WebKit somehow, eg. webkit2png, the hard part is going to be making the background transparent. Every screenshot tool I've ever tried sets the background opaque, even if the CSS background colour is transparent. The best approach might be to set the background to some unusual colour by default and then use a tool like ImageMagick to set that particular colour transparent.