Measurement of visibility of overlay content

A method of detecting a position of content within nested frames in a computer display, comprising:

• rendering, by a page renderer executed by a processor of a client device, a web page including a video block;
• determining, by the page renderer, a position of the video block with respect to the web page;
• determining, by the page renderer, a portion of the web page visible in a viewport of the client device;
• calculating, by the page renderer, a portion of the video block visible in the viewport based on the position of the video block and the portion of the web page visible in the viewport;
• passing, by the page renderer to an element renderer executed by the processor of the client device, data identifying the portion of the video block visible in the viewport;
• receiving, by the element renderer from the page renderer, the data identifying the portion of the video block visible in the viewport;
• determining, by the element renderer, a position of an overlay content item with respect to the video block;
• calculating, by the element renderer, a portion of the overlay content item visible in the viewport by comparing the position of the overlay content item and the portion of the video block visible in the viewport;
• and
• transmitting, by the element renderer, an identification of the portion of the overlay content item visible in the viewport and a timestamp to a remote server for recording an identification of overlay content item visibility.