An ad simulator having at least in-browser simulation capabilities is provided. In examples, the ad simulator may be a web browser extension, which a media developer, designer, etc. may use for ad display simulation. The browser extension may locate existing display ads on a webpage, one or more of which can be selected to be replaced with a new ad. A HyperText Transfer Protocol (HTTP) request may be made to an ad template rendering engine that returns a HyperText Markup Language (HTML) of a new ad personalized and targeted to a customer. The browser extension may then replace the existing ad with the new ad.