Template:Filter button/doc

This sets up a filter button that uses a script in MediaWiki:Common.js to show or hide content marked with Template:Filterable.

Usage:

The filter buttons work as follows:
 * If no filter buttons on the page are on, then all content in Template:Filterable will remain visible.
 * If no filter buttons in the same group are on, then the entire group is inactive. If at least one is on, the group is active.
 * The filterable items that remain visible are those which are tagged with at least one active filter from each active group. Or, to look at it the opposite way, filterable items are hidden when they do not have a tag that matches an active filter in each active filter group.
 * This means activating more filters in a single group will make more items visible. For example, if Strength and Agility are both in group Attribute, then turning on Strength and Agility shows items marked with either of them. They don't have to have both to show up.
 * But activating filters in different groups will hide more. For example, if Strength is in group Attribute, but Fire and Ice are in group Element, then turning all of those filters on will show only items marked with Strength and either Fire or Ice.

Note that when the  isn't supplied, or when   and   are the same, the filter button will display at half-opacity when it is off. Filter buttons are deactivated by default.