[1] [CITE@en[Page Visibility]]
( ([TIME[2011-07-20 07:16:55 +09:00]] 版))
<http://www.w3.org/TR/2011/WD-page-visibility-20110721/>

[2] [CITE@en[Page Visibility]]
( ([TIME[2012-07-21 04:13:20 +09:00]] 版))
<http://www.w3.org/TR/2012/CR-page-visibility-20120726/>