When a new Windows Phone Project is created, it creates the MainPage.xaml by default. This page contains the attribute
mc:Ignorable=”d”
This is an attribute which indicates to the compiler to ignore any other attributes that starts with “d”.
For example, the attributes d: DesignHeight or d: DesignWidth is a typical example of the compiler ignoring the height and width attributes.
These attributes are only used by the IDE to render the page at design time.