1. A method comprising: prior to rendering, inserting, into content code, test border code for rendering a test border around an element of content to be rendered by a content-rendering application, the element including a predetermined type of identifier; rendering the content code including the test border code; obtaining a screen capture of the rendered content; filtering the screen capture to remove all elements of the screen capture except the test border; determining a rendered dimension of the element of content using the test border; calculating a dimension difference between the rendered dimension and an expected dimension of the element of content; and responsive to the dimension difference satisfying a first predetermined condition, adjusting a first test parameter of a content testing configuration for the element of content based on the dimension difference.