What problem does this feature solve?

前端项目中有非常多的样式其实是重复的，可以通过添加新的指令，在编译代码时自动生成全局的css；开发人员甚至可以不用再写css，编译后生成全局的css，相同的属性只会生成一条css，进一步缩小vue项目的打包大小

What does the proposed API look like?

<div v-class="pd-10 fs-20 fw-700"></div>

自动生成global.css

.pd-10 { padding: 10px; }
.fs { font-size: 20px }
.fw-700 { font-weight: 700 }

scss

scss

scss只是css预处理，还是需要自己写，或者全局定义一些class；最理想的是像写内联style一样写class，自动生成css

maybe you can use windicss. vite has corresponding plugin vite-plugin-windicss

What you need is: tailwind windicss unocss

有一个css库就是类似于这样的写法

It's out of the scope of Vue core. As @kikyous pointed out there are already plenty to third party tools for your use case.