As has been said above,grammar is not a set of dead rules.

As=That which
引导非限制性定语从句,用来指代它前面的整个句子(即先行句),意思是“这一点”.这个分句可以位于句首、句中或句末.eg：
As is well known,oceans cover more than 70％ of the earth．我们知道,海洋占地球面积的百分之七十以上.