1. A method for detecting a cross-platform difference of a web application comprising: generating a first relative layout model based on a first relationship between a plurality of elements of a screen of a web application when the web application is executed on a first platform, wherein the first relative layout model indicates the first relationship and the first relationship includes a first parent and child relationship between two or more elements of the plurality of elements, a first sibling relationship between two or more elements of the plurality of elements, a first left to right order of a first left to right arrangement of two or more elements of the plurality of elements, and a first top to bottom order of a first top to bottom arrangement of two or more of the plurality of elements; generating a second relative layout model based on a second relationship between the plurality of elements of the screen of the web application when the web application is executed on a second platform, wherein the second relative layout model indicates the second relationship and the second relationship includes a second parent and child relationship between two or more elements of the plurality of elements, a second sibling relationship between two or more elements of the plurality of elements, a second left to right order of a second left to right arrangement of two or more elements of the plurality of elements, and a second top to bottom order of a second top to bottom arrangement of two or more of the plurality of elements; generating a first behavioral model of the web application when the web application is executed on the first platform, the first behavioral model including the first relative layout model and indicating, when the web application is executed on the first platform, one or more of the following: a first absolute position of one or more structural elements of the screen of the web application, a first absolute size of the one or more structural elements of the screen of the web application, a first image representation of one or more of the structural elements of the screen of the web application, a first textual representation of one or more textual elements of the screen of the web application, and a first behavior of one or more actionable elements of the screen of the web application; generating a second behavioral model of the web application when the web application is executed on the second platform, the second behavioral model including the second relative layout model and indicating, when the web application is executed on the second platform, one or more of the following: a second absolute position of the one or more structural elements of the screen of the web application, a second absolute size of the one or more structural elements of the screen of the web application, a second image representation of the one or more of the structural elements of the screen of the web application, a second textual representation of the one or more textual elements of the screen of the web application, and a second behavior of the one or more actionable elements of the screen of the web application; and determining a difference between the first behavioral model and the second behavioral model that includes determining a difference between the first relationship and the second relationship based on a comparison of the first relative layout model with respect to the second relative layout model.