fluid.css

* {box-sizing: border-box}

.main{display: flex; flex-flow: row wrap; width: 80%; margin: 0 auto}

.fluid{flex:1}

.merge2{flex:2}
.merge3{flex:3}
.merge4{flex:4}
/* add more .merge classes if you need to */

.fluid,.merge2,.merge3,.merge4 {margin:0 0 0 15px} /* optional gutter */

.clear,.columns{width: 100%}

.nested,.columns{display: flex; flex-flow: row wrap; padding:0 !important; margin:0 !important; border: 0 !important}