</span id="login">登录一下</span>
//[@id="login"]
//[text()="登录一下"]
//span
//span[1]
//span[-1]
//[starts-with(text(), '登录')]
//[contains(@id, 'ogi')]
<family><parent><child>Alice</child><child>Bob</child><child>Charlie</child></parent>
</family>
//child[text()="Bob"]/preceding-sibling::child
//child[text()="Bob"]/preceding-sibling::[1]
//child[text()="Charlie"]/preceding-sibling::
//child[text()="Bob"]/following-sibling::child
//child[text()="Bob"]/following-sibling::[1]
//child[text()="Alice"]/following-sibling::
//parent/..
