1. A computer system comprising: one or more computing devices; a hotword manager, on the one or more computing devices, configured to maintain, for each hotword of a plurality of hotwords, information that maps a unique identifier of the hotword to a visual representation of the hotword, wherein the visual representation of a first hotword of the plurality of hotwords is visually different from the visual representation of a second hotword of the plurality of hotwords; a hotword detection module, on the one or more computing devices, configured to listen for spoken input and to detect instances of spoken input that correspond to particular ones of the plurality of hotwords; and a browsing application, on the one or more computing devices, configured to: (i) receive code for an electronic document, the code including a hotword instruction that is dedicated to activating, in a presentation of the electronic document, a hotword detection capability for a particular hotword of the plurality of hotwords identified by the hotword instruction, (ii) identify the hotword instruction in the code for the electronic document, (iii) generate the presentation of the electronic document, including as a result of the hotword instruction identifying the particular hotword in the code for the electronic document: (a) activating the hotword detection capability for the particular hotword including enabling the computer system to respond to instances of spoken input that correspond to the particular hotword, and (b) selecting the visual representation of the particular hotword for display in the presentation of the electronic document, so as to indicate that the computer system is configured to respond to instances of spoken input that correspond to the particular hotword, wherein the browsing application obtains the visual representation of the particular hotword from the hotword manager using the unique identifier of the particular hotword, and (iv) perform, in response to identifying spoken input that corresponds to the particular hotword during presentation of the electronic document, a set of operations assigned to the particular hotword.