I did a CSS audit on http://en.m.wikipedia.beta.wmflabs.org/wiki/Unused_styles_test_page?mobileaction=stable (a fairly big page) with JavaScript disabled:

122 rules (46%) of CSS not used by the current page.
Many of these can be removed/added more intelligently.

Rules that need to be reviewed to see if they are relevant/can be moved elsewhee*
.references-column-count
.quotebox
caption
button
.header > .header-title
.content h1 .edit-page
.content h3 .edit-page
.content .open-block .edit-page
.content .section-heading,.content .in-block
.content .section-heading .mw-headline,.content .in-block .mw-headline
.content .section-heading > span,.content .in-block > span
.truncated-text.multi-line
.truncated-text.two-line
.heading-holder .tagline
.heading-holder .tagline:first-letter
#toc,.client-js .no-js-only,.printfooter,.jsonly

Can be deleted given they will never appear in page
#filetoc,.mw-editsection
textarea

Rules to move to gallery module
.gallery .gallerybox
.gallery .gallerybox div

Rules to standardise:
#mw-mf-page-center .mw-mf-banner

Rules to move to JavaScript:
.stable .last-modified-bar.pre-content
.last-modified-bar.active
.last-modified-bar.active #mw-mf-last-modified
.last-modified-bar.active a
.overlay #secondary-button.user-button.loading span,.header #secondary-button.user-button.loading span,.overlay .user-button.loading span,.header .user-button.loading span
#page-actions li input
#page-actions li button
.cloaked-element
.position-fixed
.no-position-fixed .drawer
.no-position-fixed .position-fixed
.touch-events :focus
.drawer,.content-overlay
.content_block
.last-modified-bar.active
.last-modified-bar.active #mw-mf-last-modified

Unknown origin should be removed/added via JavaScript whereever they are being added

.mw-echo-alert
.mw-ui-icon-collections-icon:before
div.accountcreator-show,p.accountcreator-show
span.accountcreator-show,small.accountcreator-show
table.accountcreator-show
li.accountcreator-show

Should be added conditionally e.g. dependant on namespace
.mw-mf-image-replacement
.mw-mf-special #footer,.page-Main_Page #footer
.skin-minerva .portal-column-left-wide,.skin-minerva .portal-column-left,.skin-minerva .portal-column-right,.skin-minerva .portal-column-right-narrow