Principles of writing consistent, idiomatic CSS. Here is an example of various conventions.

CSS Coding Guidelines Research

/* ==========================================================================
   Grid layout
   ========================================================================== */

/**
 * Column layout with horizontal scroll.
 *
 * This creates a single row of full-height, non-wrapping columns that can
 * be browsed horizontally within their parent.
 *
 * Example HTML:
 *
 * <div class="grid">
 *     <div class="cell cell-3"></div>
 *     <div class="cell cell-3"></div>
 *     <div class="cell cell-3"></div>
 * </div>
 */

/**
 * Grid container
 * Must only contain `.cell` children.
 */

.grid {
    height: 100%;
    /* Remove inter-cell whitespace */
    font-size: 0;
    /* Prevent inline-block cells wrapping */
    white-space: nowrap;
}

/**
 * Grid cells
 * No explicit width by default. Extend with `.cell-n` classes.
 */

.cell {
    position: relative;
    display: inline-block;
    overflow: hidden;
    box-sizing: border-box;
    height: 100%;
    /* Set the inter-cell spacing */
    padding: 0 10px;
    border: 2px solid #333;
    vertical-align: top;
    /* Reset white-space */
    white-space: normal;
    /* Reset font-size */
    font-size: 16px;
}

/* Cell states */

.cell.is-animating {
    background-color: #fffdec;
}

/* Cell dimensions
   ========================================================================== */

.cell-1 { width: 10%; }
.cell-2 { width: 20%; }
.cell-3 { width: 30%; }
.cell-4 { width: 40%; }
.cell-5 { width: 50%; }

/* Cell modifiers
   ========================================================================== */

.cell--detail,
.cell--important {
    border-width: 4px;
}