demo.css

.element-item {
    position: relative;
    float: left;
    width: 100px;
    height: 100px;
    margin: 5px;
    padding: 10px;
    background: #888;
    color: #262524;
}

.element-item > * {
    margin: 0;
    padding: 0;
}

.element-item .name {
    position: absolute;

    left: 10px;
    top: 60px;
    text-transform: none;
    letter-spacing: 0;
    font-size: 12px;
    font-weight: normal;
}

.element-item .symbol {
    position: absolute;
    left: 10px;
    top: 0px;
    font-size: 42px;
    font-weight: bold;
    color: white;
}

.element-item .number {
    position: absolute;
    right: 8px;
    top: 5px;
}

.element-item .weight {
    position: absolute;
    left: 10px;
    top: 76px;
    font-size: 12px;
}

.element-item.alkali          { background: #F00; background: hsl(   0, 100%, 50%); }
.element-item.alkaline-earth  { background: #F80; background: hsl(  36, 100%, 50%); }
.element-item.lanthanoid      { background: #FF0; background: hsl(  72, 100%, 50%); }
.element-item.actinoid        { background: #0F0; background: hsl( 108, 100%, 50%); }
.element-item.transition      { background: #0F8; background: hsl( 144, 100%, 50%); }
.element-item.post-transition { background: #0FF; background: hsl( 180, 100%, 50%); }
.element-item.metalloid       { background: #08F; background: hsl( 216, 100%, 50%); }
.element-item.diatomic        { background: #00F; background: hsl( 252, 100%, 50%); }
.element-item.halogen         { background: #F0F; background: hsl( 288, 100%, 50%); }
.element-item.noble-gas       { background: #F08; background: hsl( 324, 100%, 50%); }

/* varying sizes */
.is-varying-sizes .element-item.width2 { width: 210px; }
.is-varying-sizes .element-item.height2 { height: 210px; }