Several pages have both an input and a select element with html id 'hostid'. According to the html spec hostid's should be unique.
While not generally a problem it can cause unexpected behaviour and odd bugs while developing new code (as noticed here: https://support.zabbix.com/browse/ZBXNEXT-2185?focusedCommentId=115362&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-115362) so I think it would be good to fix this.
Pages of interest:
Configuration/Hosts entities: Graphs, Web, Triggers, Applications.