*{
    box-sizing: border-box;
}

header{
    width: 90%;
    margin: 20px auto;
    font-size: 4vw;
    color: grey;
    border-bottom: 10px double grey;
}

article,footer,main{
    width: 90%;
    margin: 20px auto;
}

ul{
    display: flex;
    flex-wrap: nowrap;
}

li{
    list-style-type: none;
    width: 25%;
    font-size: 1.2vw;
    padding: 1px 10px;
    border-right: 1px solid black;
}

li:last-of-type{
    border:none;
}

li > img {
    margin:0px 2px;
}
span{
    color: lightblue;
}
footer{color: grey;
}

main{
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
}

img[alt="principal"],section{
    width: 90%;
}
h2{
    width: 100%;
    font-size: 2vw;
}
section{
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
}

section > img{
    width: 50%;
    border:1px solid grey;
    margin: 10px 0px;

}
img[alt="stu"],img[alt="twi"],img[alt="ema"]{
    display: none;
}
@media screen and (min-width: 576px){

    img[alt="stu"]{
        display: inline;
    }
    

    section{
        justify-content: center;
    }
section > img{
    width: 33%;
    border:  1px solid grey;
    margin: 10px 0px;
}
}

@media screen and (min-width: 1000px){

img[alt="twi"],img[alt="ema"]{
display: inline;
}
main{
    flex-wrap: nowrap;
    justify-content: space-between;
}

section > img{
    width: 50%;
    border:  1px solid grey;
    margin: 10px 0px;
}
section > h2{
    padding: 10px 0px;
}
}