%%::quote-start%% and %%::quote-end%%, to match characters with the Quotation_Mark property that are direct children of the element and that are immediately inside the start and end (respectively) of the element, ignoring White_Space characters. (This would help a lot with <q> in HTML5.)
[:placeholder](https://developer.mozilla.org/en/CSS/%3a-moz-placeholder) - matches form elements displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you’ve changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.
to select cells in a column: %%col.foo // td%% – done with td:column(col.foo)
to select labels applying to an input control: %%input // label%% – done without further knowledge of markup language, i.e. $label /for/ input, $label input
to select map elements associated with an image: %%img // map%% – opposite of input–label situation, img /usemap/ map
to select arbitrary associated elements: %%x[href#] // y[id#]%% – always any ID in target, x /href/ y
only-child-parent Most use cases for the parent selector care only about the only child. This doesn’t have the same perf implications as a general parent selector. – solved with :matches() and $