Where is the class "placeholder" defined?
The only uses of the placeholder class in core are (filename, followed by line number, follby the line):

modules/dashboard/dashboard.js:35:        if ($('.placeholder', this).length == 0) {
modules/dashboard/dashboard.js:38:        $('.placeholder', this).html(empty_text);
modules/dashboard/dashboard.js:41:        $('.placeholder', this).remove();
modules/system/system.admin.css:264:.exposed-filters .current-filters .placeholder {


and also a few in jquery.ui that you can ignore:

misc/ui/jquery.ui.draggable.min.js:40
misc/ui/jquery.ui.sortable.min.js:26
misc/ui/jquery.ui.sortable.min.js:27
misc/ui/jquery.ui.sortable.min.js:29
misc/ui/jquery.ui.sortable.min.js:39
misc/ui/jquery.ui.sortable.min.js:40
misc/ui/jquery.ui.sortable.min.js:43
misc/ui/jquery.ui.sortable.min.js:55
misc/ui/jquery.ui.sortable.min.js:56
misc/ui/jquery.ui.sortable.min.js:59
misc/ui/jquery.ui.sortable.min.js:60


You can safely ignore all this and just implement your own css for the .placeholder class in your theme.