Separate all styling to existing CSS file.
Organize CSS in separate directory.
Convert to Google's CSS style guide for consistency.
Remove a lingering style declaration in options.html.
Adhere to CSS Lint: use class instead of ID for style declaration.
Google style guide adherence for consistency (indentation, consistent
quotation marks, etc.)
HTML5 migration: self-closing tags not needed, specify charset, empty
attributes are implied, etc.
Use HTML5 form methods for input validation in combination with JS.

Allow initial empty values on intial extension load. Use placeholders,
patterns and titles to guide the user. Allow user to be left blank if
single-user mode is enabled.
Use a better field type for update interval.
Capitalize acronyms.
Change status message handling, drop external libs.

The external libs weren't working with Google's new security practices
anyway. Probably fixable by digging deeper, but losing them altogether
is cleaner. HTML5 takes care of the input validation and usability in
the form anyway (actually, the JS error messages should never be shown
if the HTML limits work as they should).
Nicer presentation when items/feeds have not yet been updated.

Also use purer DOM methods instead if innerHTML to silence the JS error
console in Chrome when selecting tags.
Enforce consistent variable and class naming.

Introduce localStorage migration function with note on removing it in
future updates.
Remove antique note on minimum version requirements.
Clearer update definition messages.

I know I wondered over that when I first looked at the extension, at
least.
See last commit; make it even clearer.
Remove unnecessary div tags. More consistent class naming.

Add alt and title attributes to the logo.
Popup does not seem to have been used for some time, remove.
Settle on px for positioning. Move submit button in line with input fields.

Also fix a class naming that didn't get staged last time.
Remove executable permission from images.