:-moz-placeholder
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represents any form element displaying placeholder text.This allows web developers and theme desgners to customize the appearance of placeholder text,which is a light grey color by default.This may work well if you've changed the background color of your form fields to be a similar color,for example,so you can use this pseudo-class to change the placeholder text color.
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它是一个伪类，是用来设置表单文本框的占位文本的样式，其默认是一个浅灰色。比如当设置颜色相似的时候，为了使其正常显示，可以用它来解决
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input:-moz-placeholder{
color:green;
}
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在mozilla的官网上我们可以看到：introduced in Gecko 2.0(Firefox 4|Thunderbird 3.3|SeaMonkey 2.1)
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扩展阅读：
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1、https://developer.mozilla.org/en/CSS/:-moz-placeholder
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