1. A method for automatic detection of Cascading Style Sheet (CSS) conflicts, the method comprising: receiving, by one or more processors, at least one user interface (UI) constraint and at least one first CSS rule defined by a developer of a widget as well as at least one second CSS rule customized by a user of the widget, said UI constraints being used to restrict the UI of said widget; running, by one or more processors, a page containing said widget; performing, by one or more processors, automatic validation on each UI constraint based on a result of running the page; and in response to deriving a UI constraint being violated from said validation, finding, by one or more processors, one or more of the second CSS rules that lead to violation of the UI constraint in a first set made up of the second CSS rules that override the first CSS rules.