A:A 元素利用 float 的包裹性，B 元素则利用 BFC。
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Reference:Expand a div to fill the remaining width
2.Meaning of ~ in import of scss files
From documentation on a
webpack provides an advanced mechanism to resolve files. The sass-loader uses node-sass' custom importer feature to pass all queries to the webpack resolving engine. Thus you can import your Sass modules from node_modules. Just prepend them with a ~ to tell webpack that this is not a relative import
So if you have a file named foo.css and a module foo then you would use ~ if you want to include the module.
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