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https://iqfuyunw.wordpress.com/2019/11/17/css-background-size%e4%b8%8e%e8%83%8c%e6%99%af%e5%9b%be%e7%89%87%e5%a1%ab%e6%bb%a1div%e7%9a%84%e5%85%b7%e4%bd%93%e7%a0%94%e7%a9%b6%e5%8c%85%e6%8b%accover-contain%e5%b1%9e%e6%80%a7%e6%b5%8b%e8%af%95/
css background-size与背景图片填满div的具体研究包括cover contain属性测试