1. A computerized method of generating a content display area on a web page, the method comprising: receiving, by a computer server, a first request to insert a content slot in a web page loaded at a web browser for viewing on a client device; retrieving, by the computer server from a document object model (DOM) document of the web page, position data of a plurality of objects within the web page; determining, by the computer server, a plurality of eligible content slot positions on the web page based on the position data of the plurality of objects; modifying, by the computer server, the DOM document to insert a plurality of placeholders, each placeholder corresponding to a respective one of the plurality of eligible content slot positions within the webpage and including a respective selectable actionable item, which when selected, causes the client device to transmit a second request to the computer server to insert a content slot at the content slot position corresponding to that placeholder; displaying, via the browser, a version of the web page corresponding to the modified DOM document, the version of the webpage including the plurality of placeholders; receiving, by the computer server from the client device, responsive to a first user input indicative of a selection of an actionable item of a first placeholder of the plurality of placeholders, a third request to insert a first content slot at a first content slot position corresponding to the first placeholder; responsive to receiving the third request, computing, by the computer server, at least one dimension of the first placeholder based on the first content slot position relative to positions of the plurality of objects; modifying, by the computer server, the DOM documents to i) adjust the size of the first placeholder using the at least one dimension computed and to ii) remove the remaining inserted placeholders for other eligible content slot positions, the web browser loading the first placeholder with the respective size adjusted and the plurality of objects for viewing; receiving, by the computer server, a second user input indicating confirmation of the first content slot position on the web page; generating, by the computer server, machine-readable instructions for inserting the first content slot at the first content slot position corresponding to the first placeholder on the web page based on the confirmation, the first content slot having a dimension corresponding to the at least one dimension computed; and providing, by the computer server, display data representative of the machine-readable instructions.