We use a number of CSS classes as "helpers" to aid us with repetitive tasks or commonly-encountered problems. Have a look at the table below for examples of where this happens.

CSS selector Explanation / usage
.js-hidden Used on things which should be hidden for JavaScript-enabled browsers. For example, we have a dropdown list of sections which is triggered via JS - we make this list .js-hidden so that users without JS can see it, but users with JS only see it when it's activated (and the class is removed).
.js-visible Used to mark things which should only appear for JavaScript-enabled browsers. For example, we set the autorefresh controls for liveblogs as .js-visible as these tools require JS to work.
.is-off Similar to .js-hidden, but indicates content which will be hidden for all browsers on pageload (via CSS) and later shown via JS. In other cases, consider using .js-hidden, which means elements will be shown by default to non-JS users (whereas .is-off content will never be seen by non-JS users).
.is-on Sets display: block on an element. Used for JS toggles.
.maxed Gives an element width: 100%. Useful for images.
.pull-left / .pull-right Floats elements to the left/right respectively. Doesn't add margins or similar.
.margin-top / .margin-bottom Adds margin properties for generic baseline-based vertical spacing. We try to use margin-bottom everywhere, where possible, to avoid confusion.
.border-top / .border-bottom Add a standard border below/above the element. Doesn't add padding/margin.
.h An accessible way to hide content (eg, image replacement, rather than using text-indent: -1000em).
.cf Clearfix (as written by Nicolas Gallagher).