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当使用Selenium进行脚本自动化时，我尝试使用css选择器查找所需的web元素，然后使用firebug获取该元素的唯一css路径。然而，我很惊讶，因为有时萤火虫给我一个不正确的CSS路径，虽然它确实与硒一起工作。萤火虫如何识别硒的独特css路径？
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<span class="postdetails">20/03/2016 13:58:24<br>
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<span class="postdetails">20/03/2016 14:34:25<br>
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我需要的跨度，其在第三个tr名下是“bg_small_yellow”。然而，萤火虫给我独特的CSS路径为“tr.bg_small_yellow：n-child（4）> td：n-child（2）.topictitle> a：n-child（1）”，你看到只有3总共bg_small_yellow。第四个在哪里？它是如何工作的？
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