IFrames are commonly used to allow embedding of third-party content. Many popular services, such as YouTube and Facebook, make extensive use of IFrames in their embeddable widgets.
Can have unknown width and height
Can have unknown width/height ratio
Can change its width or height as users interact with dynamic content and we want the size of the IFrame to change accordingly to avoid scrollbars.
Making IFrames with such constraints is a challenging task. We developed this tiny (0.9KB minified and gzipped) jQuery plugin, at NPR, to make the task easier.
For simpler use-cases, e.g. targeting the embedding of fixed ratio video content, you may want to check-out: FitVid.js, instead.