header{background-color: greenyellow;}
nav{background-color: magenta;}
main{background-color: darkblue;}
section{background-color: aqua;}
article{background-color: orange;width: 50%; margin: 2rem;}
aside{background-color: grey;}
footer{background-color: darkcyan;}

.contenedor1 {display: flex;}
.a3 {width: 75%;}
.a4 {width: 25%;}