CSS rules optimized for site readability

readability.css

:root {
    --bg-color: #ffffff;
    --font-color: #000000;
    /* highest contrast colors
    for light and dark themes */
    --red: #ec0000;
    --green: #008900;
    --blue: #5f5fff;
    --gray: #757575;
}

@media (prefers-color-scheme: dark) {
    :root {
        /* change to dark theme */
        --bg-color: #000000;
        --font-color: #ffffff;
    }
}

*:not(li, progress, span) {
    border-radius: 5px;
    /* no overflowing body */
    max-width: 100%;
    overflow: auto;
}

*:disabled {
    cursor: not-allowed !important;
}

[href],
dfn {
    /* no visited color */
    color: var(--blue);
}

[href]:hover {
    text-decoration: none;
}

[href^="mailto:"]::before {
    content: "📧 ";
}

abbr {
    cursor: help;
}

abbr,
var {
    color: var(--red);
}

blockquote {
    /* add bar before quote */
    border-left: 0.3em solid var(--gray);
    padding-left: 1em;
}

body {
    /* high contrast */
    background: var(--bg-color);
    color: var(--font-color);
    /* most readable wed-safe font */
    font-family: Helvetica;
    /* 12pt is minimum */
    font-size: 14pt;
    /* required with justified text */
    hyphens: auto;
    /* experimental:
    highest quality images */
    image-rendering: high-quality;
    /* 1.5 is minimum */
    line-height: 1.6;
    /* center body in page */
    margin: auto;
    /* space inside body */
    padding: 0 1em;
    /* each line is similar length */
    text-align: justify;
    /* browser focuses on readability */
    text-rendering: optimizeLegibility;
    /* line length of 60 characters
    (between recommended 45-80)
    & subtract the padding */
    width: min(60ch, calc(100% - 2em));
}

button,
input[type="button"],
input[type="reset"],
input[type="submit"] {
    border: 1px solid var(--gray);
    /* appear clickable */
    cursor: pointer;
}

button:hover,
input[type="button"]:hover,
input[type="reset"]:hover,
input[type="submit"]:hover {
    /* make border blend into background */
    border: 1px solid var(--bg-color);
}

code {
    /* classic green code */
    color: var(--green);
}

figure {
    /* center caption under image */
    text-align: center;
}

footer,
header {
    margin-top: 1em;
    text-align: center;
}

html {
    /* better for jump scrolling */
    scroll-behavior: smooth;
}

iframe {
    /* common screen ratio */
    aspect-ratio: 3/2;
    /* keep from overflowing */
    width: 99%;
}

kbd {
    /* appear like a key */
    box-shadow: 1px 1px 2px 1px;
}

nav {
    display: flex;
    /* space links apart */
    justify-content: space-around;
}

small {
    /* decrease visibility */
    color: var(--gray);
}

summary {
    /* indicate interaction */
    cursor: pointer;
    font-weight: bold;
}

table {
    /* multiple borders merge */
    border-collapse: collapse;
    display: block;
}

tbody>tr:nth-child(odd) {
    /* set background of odd cells */
    background: var(--gray);
}

td,
th {
    border: 1px solid;
    border-collapse: collapse;
}

This style sheet is designed to make any HTML appear very readable using the fewest CSS rules.

I used the HTML5 Test Page to verify readability in every situation and backed each rule with relevant research. Specifically, the body tag increases readability / WPM with:

1. the fastest web-safe font,
2. size 14pt font,
3. nearly double line spacing,
4. legibility-optimized rendering,
5. and a maximum of 60 characters per line.

Here is a before and after comparison of the stylesheet.