The funniest part is that the developers of the server most of the time do not care much about the standard but only whether it works with vscode. To give you an example, when receiving the completion results the client is supposed to calculate the prefix and filter against it. In html file if you have <foo it will calculate the prefix foo but if you are in xml file and you want to complete <foo the prefix to match against is <foo. So, if we want to work against these servers we have to implement the vscode's bizarre behaviour.