11. A non-transitory computer-readable storage device encoded with a computer program product, the computer program product including instructions that, when executed, perform operations comprising: generating, by a server system, a markup language document for a webpage, the markup language document comprising JavaScript code, a comment tag, a variable, and an attribute that is separate from the variable and that is associated with a second markup language tag other than the comment tag, the comment tag being separate from the JavaScript code and arranged to indicate particular content in the markup language document that is not to be processed for display when the comment tag is interpreted as a comment tag by a first computing device that is caused to execute the markup language document; wherein the markup language document is structured to cause the first computing device, when the markup language document is executed by the first computing device and in response to the variable being determined to hold a first value, (i) to render a first version of the webpage, wherein the rendering includes using the JavaScript code to write the second markup language tag in the markup language document such that the characters that form the comment tag are set as a value of the attribute and the comment tag is ignored as a comment tag as a result of the first computing device being caused to interpret the characters that form the comment tag as the value of the attribute, and (ii) to provide, to the server system for monitoring a conversion rate of the webpage, first conversion data that indicates that the first version of the webpage has been rendered, and wherein the markup language document is further structured to cause the first computing device, when the markup language document is executed by the first computing device and in response to the variable being determined to hold a second value that is different from the first value, (i) to render a second version of the webpage, wherein the rendering includes interpreting the comment tag normally such that the particular content in the markup language document indicated by the comment tag is not processed by the first computing device for display, and (ii) to provide, to the server system for monitoring a conversion rate of the webpage, second conversion data that indicates that the second version of the webpage has been rendered; and transmitting the markup language document to the first computing device.
11. A non-transitory computer-readable storage device encoded with a computer program product, the computer program product including instructions that, when executed, perform operations comprising: generating, by a server system, a markup language document for a webpage, the markup language document comprising JavaScript code, a comment tag, a variable, and an attribute that is separate from the variable and that is associated with a second markup language tag other than the comment tag, the comment tag being separate from the JavaScript code and arranged to indicate particular content in the markup language document that is not to be processed for display when the comment tag is interpreted as a comment tag by a first computing device that is caused to execute the markup language document; wherein the markup language document is structured to cause the first computing device, when the markup language document is executed by the first computing device and in response to the variable being determined to hold a first value, (i) to render a first version of the webpage, wherein the rendering includes using the JavaScript code to write the second markup language tag in the markup language document such that the characters that form the comment tag are set as a value of the attribute and the comment tag is ignored as a comment tag as a result of the first computing device being caused to interpret the characters that form the comment tag as the value of the attribute, and (ii) to provide, to the server system for monitoring a conversion rate of the webpage, first conversion data that indicates that the first version of the webpage has been rendered, and wherein the markup language document is further structured to cause the first computing device, when the markup language document is executed by the first computing device and in response to the variable being determined to hold a second value that is different from the first value, (i) to render a second version of the webpage, wherein the rendering includes interpreting the comment tag normally such that the particular content in the markup language document indicated by the comment tag is not processed by the first computing device for display, and (ii) to provide, to the server system for monitoring a conversion rate of the webpage, second conversion data that indicates that the second version of the webpage has been rendered; and transmitting the markup language document to the first computing device. 12. The non-transitory computer-readable storage device of claim 11 , wherein the JavaScript code defines the variable to hold the value.