MDN’s new design is in Beta! A sneak peek:

Type: space-separated list of attribute names
A space-separated list of attributes that are maintained when the window is closed. When the window is re-opened, the values of persistent attributes are restored. In Mozilla, persistent attributes are stored in the per-profile file xulstore.json. Persistence can also be stored using the document.persist function. In order for persistence to work, the element must also have an id. Persistence will not remember the absence of an attribute, so for boolean attributes like checked where absence means false, you will need to explicitly set the attribute to false before the window closes (bug 15232).

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 Last updated by: V@no,