23. One or more non-transitory computer-readable devices including instructions that, when executed by one or more processors, cause performance of operations, the operations comprising: receiving, by a client computing device and as having been sent from a server system, a markup language document for a webpage that includes 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 by the client computing device for display when the comment tag is interpreted as a comment tag by the client computing device; determining that the variable holds a first value; in response to determining that the variable holds the first value, rendering a first version of the webpage that includes a representation of the particular content for display, 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 client computing device interpreting the characters that form the comment tag as the value of the attribute; and providing, 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, wherein the markup language document is structured to cause the client computing device, in response to the variable holding a second value that is different from the first value, (i) to render a second version of the webpage that does not include the representation of the particular content for display, the comment tag being arranged for interpretation as a comment tag in the second version of the webpage, and (ii) to provide, to the server system for monitoring the conversion rate of the webpage, second conversion data that indicates that the second version of the webpage has been rendered.
23. One or more non-transitory computer-readable devices including instructions that, when executed by one or more processors, cause performance of operations, the operations comprising: receiving, by a client computing device and as having been sent from a server system, a markup language document for a webpage that includes 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 by the client computing device for display when the comment tag is interpreted as a comment tag by the client computing device; determining that the variable holds a first value; in response to determining that the variable holds the first value, rendering a first version of the webpage that includes a representation of the particular content for display, 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 client computing device interpreting the characters that form the comment tag as the value of the attribute; and providing, 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, wherein the markup language document is structured to cause the client computing device, in response to the variable holding a second value that is different from the first value, (i) to render a second version of the webpage that does not include the representation of the particular content for display, the comment tag being arranged for interpretation as a comment tag in the second version of the webpage, and (ii) to provide, to the server system for monitoring the conversion rate of the webpage, second conversion data that indicates that the second version of the webpage has been rendered. 26. The one or more non-transitory computer-readable devices of claim 23 , wherein the markup language document further comprises code that is operable to determine the value of the variable.