mobile.scss

/* http://meyerweb.com/eric/tools/css/reset/ 
   v2.0 | 20110126
   License: none (public domain)
*/

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
	margin: 0;
	padding: 0;
	border: 0;
	font-size: 100%;
	font: inherit;
	vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure, 
footer, header, hgroup, menu, nav, section {
	display: block;
}
body {
	line-height: 1;
}
ol, ul {
	list-style: none;
}
blockquote, q {
	quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
	content: '';
	content: none;
}
table {
	border-collapse: collapse;
	border-spacing: 0;
}


/* START: My CSS */

header {
    background-image: url("image-header.jpg");
    position: relative;
    top: 0;
    left: 0;
    right: 0;
    background-repeat: no-repeat;
    background-size: cover;
    padding-top: 20%;
}

#main {
    margin: 0 auto;
    max-width: 1500px;
}

li {
    display: block;
    margin: 1%;
    background-color: purple;
}

.gallery-image {
    display: block;
    border: 5px solid red;
    width: 100%;
    /* boder-box effect: width or height includes border padding and content */
    box-sizing: border-box;
}
    li:nth-child(2) .gallery-image {
       border: 5px solid #333;
    }

footer {
    background-color: #333;
    height: 300px;
}

@media only screen and (min-width: 740px) {
    
    body {
        background-color: green;
    }
    
    #gallery {
        overflow: hidden;
    }
    
    li {
        width: 48%;
        float: left;
    }
    
}

@media only screen and ( min-width: 980px ) {
    
    body {
        background-color: yellow;
    }
    
    li {
        width: 31.33%;
    }
    
@media only screen and ( min-width: 1200px ) {
    
    body {
        background-color: lightcoral;
    }
    
    li {
        width: 23%;
    }
}