A computer-implemented method for allowing a user to specify a responsive web page design in a graphical design tool comprising: displaying a rendering of a first dimension version of said web page for a user, said rendering including an interactive widget of a plurality of interactive widgets (702, 703) of said web page; receiving an inheritance structure specification from said user through an inheritance specification user interface, said inheritance structure specification defining an inheritance structure in which a second dimension version of said web page is defined as a child of said first dimension version; receiving a widget property specification for a first property of said interactive widget from said user while said user is editing said first dimension version; the inheritance specification user interface displaying a graphical representation of said inheritance structure; and storing said widget property specification for said first dimension version and said second dimension version while said user is editing said first dimension version; wherein said widget property specification would not have been stored for said first dimension version if said widget property specification had been received while said user was editing said second dimension version; said inheritance specification user interface enables said user to select and edit said first or second dimension version; receiving said inheritance structure specification from said user modifies a first inheritance structure; said first inheritance structure defines said first dimension version as a child of said second dimension version; and said first dimension version being associated with a relevant rendering space dimension that is smaller than a respective relevant rendering space dimension that said second dimension version is associated with. displaying a rendering of a first dimension version of said web page for a user, said rendering including an interactive widget of a plurality of interactive widgets (702, 703) of said web page; receiving an inheritance structure specification from said user through an inheritance specification user interface, said inheritance structure specification defining an inheritance structure in which a second dimension version of said web page is defined as a child of said first dimension version; receiving a widget property specification for a first property of said interactive widget from said user while said user is editing said first dimension version; the inheritance specification user interface displaying a graphical representation of said inheritance structure; and storing said widget property specification for said first dimension version and said second dimension version while said user is editing said first dimension version; wherein said widget property specification would not have been stored for said first dimension version if said widget property specification had been received while said user was editing said second dimension version; said inheritance specification user interface enables said user to select and edit said first or second dimension version; receiving said inheritance structure specification from said user modifies a first inheritance structure; said inheritance specification user interface enables said user to select and edit said first or second dimension version; receiving said inheritance structure specification from said user modifies a first inheritance structure; said first inheritance structure defines said first dimension version as a child of said second dimension version; and said first dimension version being associated with a relevant rendering space dimension that is smaller than a respective relevant rendering space dimension that said second dimension version is associated with.