from https://premium.wpmudev.org/blog/add-masonry-grid-layouts-to-your-wordpress-site-with-just-css/

masonry.css

/* Masonry Custom CSS  */

/* Masonry container */
body.blog div#content, body.archive div#content {
-moz-column-count: 4;
-webkit-column-count: 4;
column-count: 4;
-moz-column-gap: 1em;
-webkit-column-gap: 1em;
column-gap: 1em;
}

/* Masonry bricks or child elements */

body.blog article, body.archive article {
background-color: #eee;
display: inline-block;
margin: 0 0 1em;
padding: 1em;
width: 100%;
}

body.archive .archive-header, body.blog .paging-navigation, body.archive .paging-navigation {
background-color: #ffffff;
-webkit-column-span: all;
column-span: all;
}

h1, h2, h3, h4, h5, h6, a {
-ms-word-wrap: break-word;
word-wrap: break-word;
}

@media only screen and (max-width : 1024px) {

body.blog div#content, body.archive div#content { /* Masonry container */
-moz-column-count: 3;
-webkit-column-count: 3;
column-count: 3;
}
}

@media only screen and (max-device-width : 1024px) and (orientation : portrait) {

body.blog div#content, body.archive div#content { /* Masonry container */
-moz-column-count: 2;
-webkit-column-count: 2;
column-count: 2;
}
}

@media only screen and (max-width : 768px) {

body.blog div#content, body.archive div#content { /* Masonry container */
-moz-column-count: 2;
-webkit-column-count: 2;
column-count: 2;
}
}

@media only screen and (max-width : 480px) {

body.blog div#content, body.archive div#content { /* Masonry container */
-moz-column-count: 1;
-webkit-column-count: 1;
column-count: 1;
}
}