Because there is an organisation named W3C (World Wide Web Consortium) that makes standards for how the web is supposed to be coded - if people actually bothered following the standards, there wouldn't be thousands of badly coded webpages out there that breaks in other browsers than specific versions of Internet Explorer etc.
Opera and Firefox are good examples of browsers that do a decent job of pertaining to the standards.
Deprecated tags and inline styling of HTML-elements is preferably to be avoided. Inline styling is perfectly valid use of the "style"-tag, but should preferably be put in an external CSS-file, so as to simplify the HTML and ease the changing of styles at later stages.
Styling of a RSS-feed should not be done, simple as that. This is not something WordPress has done - they've just taken away the automatic support for a deprecated styling-tag, and that is a good thing. Of course people who's gotten used to the tag complains, and want it back - I for one don't. There are probably other ways of getting this through in the feeds, but I don't see the point - an RSS feed is not supposed to look like the webpage - nor the post itself, for that matter. An RSS-feed is supposed to take the content of the site, and present it in a simple, text-only matter.
I do not remember the original specification right now, but I wonder if there wasn't even an initial support for images in the feeds, although I might be mistanken regarding that.