https://blog.faceutil.com/2017/04/26/htmlcss-a-link-a-visited-a-active-a-hover-%ec%82%ac%ec%9a%a9%eb%b2%95-%ec%9e%90%ec%84%b8%ed%9e%88-%ec%95%8c%ea%b8%b0-%e2%99%ad/