Can't wait until homebrew is updated to try this out. A part of me is morbidly curious to try out the new web browser. This may be the impetus I've been needing to make the jump to GNUs for my email.
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The HTML rendering engine (shr) was originally written as part of the Gnus project for exactly that task: To render HTML in RSS and email. The browser (eww) just adds http support via url.el and a user front end.