Correcting <sup> & <sub> line-height

Question

In Normalize.css, is position relative really necessary for <sup> & <sub> to fix the line-height issue?

/**
 * Prevent `sub` and `sup` affecting `line-height` in all browsers.
 */

sub,
sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline;
}

sup {
  top: -0.5em;
}

sub {
  bottom: -0.25em;
}

What's wrong with using the browser default vertical-align: sup/sub?

Or in other words, why not simply declare sub, sup { line-height: 0; } instead?

Answer 1

Let's look at the idea behind normalize.css. From the normalize.css Github page:

Normalize.css makes browsers render all elements more consistently and in line with modern standards. It precisely targets only the styles that need normalizing.

The main keyword to take from this statement is "consistently".

In these screenshots, the top line has line-height: 0 and font-size: 75% for sub and sup. The bottom line has the normalize CSS and is more consistently positioned between browsers.

IE6

IE8

IE11

Chrome — The differences are not as big in modern browsers.

The differences are minor, but the normalize CSS gives a more consistent look between browsers, including IE6. This consistency has been achieved using position: relative.

This is what I used for the screenshots

Also as a jsBin example. IE screenshots taken with the jsbin link inserted into netrenderer.com

body {
  background: #CCC;
}

p {
  background: #F00;
}









/*! normalize.css v3.0.2 | MIT License | git.io/normalize */

/**
 * 1. Set default font family to sans-serif.
 * 2. Prevent iOS text size adjust after orientation change, without disabling
 *    user zoom.
 */

html {
  font-family: sans-serif; /* 1 */
  -ms-text-size-adjust: 100%; /* 2 */
  -webkit-text-size-adjust: 100%; /* 2 */
}

/**
 * Remove default margin.
 */

body {
  margin: 0;
}

/* HTML5 display definitions
   ========================================================================== */

/**
 * Correct `block` display not defined for any HTML5 element in IE 8/9.
 * Correct `block` display not defined for `details` or `summary` in IE 10/11
 * and Firefox.
 * Correct `block` display not defined for `main` in IE 11.
 */

article,
aside,
details,
figcaption,
figure,
footer,
header,
hgroup,
main,
menu,
nav,
section,
summary {
  display: block;
}

/**
 * 1. Correct `inline-block` display not defined in IE 8/9.
 * 2. Normalize vertical alignment of `progress` in Chrome, Firefox, and Opera.
 */

audio,
canvas,
progress,
video {
  display: inline-block; /* 1 */
  vertical-align: baseline; /* 2 */
}

/**
 * Prevent modern browsers from displaying `audio` without controls.
 * Remove excess height in iOS 5 devices.
 */

audio:not([controls]) {
  display: none;
  height: 0;
}

/**
 * Address `[hidden]` styling not present in IE 8/9/10.
 * Hide the `template` element in IE 8/9/11, Safari, and Firefox < 22.
 */

[hidden],
template {
  display: none;
}

/* Links
   ========================================================================== */

/**
 * Remove the gray background color from active links in IE 10.
 */

a {
  background-color: transparent;
}

/**
 * Improve readability when focused and also mouse hovered in all browsers.
 */

a:active,
a:hover {
  outline: 0;
}

/* Text-level semantics
   ========================================================================== */

/**
 * Address styling not present in IE 8/9/10/11, Safari, and Chrome.
 */

abbr[title] {
  border-bottom: 1px dotted;
}

/**
 * Address style set to `bolder` in Firefox 4+, Safari, and Chrome.
 */

b,
strong {
  font-weight: bold;
}

/**
 * Address styling not present in Safari and Chrome.
 */

dfn {
  font-style: italic;
}

/**
 * Address variable `h1` font-size and margin within `section` and `article`
 * contexts in Firefox 4+, Safari, and Chrome.
 */

h1 {
  font-size: 2em;
  margin: 0.67em 0;
}

/**
 * Address styling not present in IE 8/9.
 */

mark {
  background: #ff0;
  color: #000;
}

/**
 * Address inconsistent and variable font size in all browsers.
 */

small {
  font-size: 80%;
}



/* Embedded content
   ========================================================================== */

/**
 * Remove border when inside `a` element in IE 8/9/10.
 */

img {
  border: 0;
}

/**
 * Correct overflow not hidden in IE 9/10/11.
 */

svg:not(:root) {
  overflow: hidden;
}

/* Grouping content
   ========================================================================== */

/**
 * Address margin not present in IE 8/9 and Safari.
 */

figure {
  margin: 1em 40px;
}

/**
 * Address differences between Firefox and other browsers.
 */

hr {
  -moz-box-sizing: content-box;
  box-sizing: content-box;
  height: 0;
}

/**
 * Contain overflow in all browsers.
 */

pre {
  overflow: auto;
}

/**
 * Address odd `em`-unit font size rendering in all browsers.
 */

code,
kbd,
pre,
samp {
  font-family: monospace, monospace;
  font-size: 1em;
}

/* Forms
   ========================================================================== */

/**
 * Known limitation: by default, Chrome and Safari on OS X allow very limited
 * styling of `select`, unless a `border` property is set.
 */

/**
 * 1. Correct color not being inherited.
 *    Known issue: affects color of disabled elements.
 * 2. Correct font properties not being inherited.
 * 3. Address margins set differently in Firefox 4+, Safari, and Chrome.
 */

button,
input,
optgroup,
select,
textarea {
  color: inherit; /* 1 */
  font: inherit; /* 2 */
  margin: 0; /* 3 */
}

/**
 * Address `overflow` set to `hidden` in IE 8/9/10/11.
 */

button {
  overflow: visible;
}

/**
 * Address inconsistent `text-transform` inheritance for `button` and `select`.
 * All other form control elements do not inherit `text-transform` values.
 * Correct `button` style inheritance in Firefox, IE 8/9/10/11, and Opera.
 * Correct `select` style inheritance in Firefox.
 */

button,
select {
  text-transform: none;
}

/**
 * 1. Avoid the WebKit bug in Android 4.0.* where (2) destroys native `audio`
 *    and `video` controls.
 * 2. Correct inability to style clickable `input` types in iOS.
 * 3. Improve usability and consistency of cursor style between image-type
 *    `input` and others.
 */

button,
html input[type="button"], /* 1 */
input[type="reset"],
input[type="submit"] {
  -webkit-appearance: button; /* 2 */
  cursor: pointer; /* 3 */
}

/**
 * Re-set default cursor for disabled elements.
 */

button[disabled],
html input[disabled] {
  cursor: default;
}

/**
 * Remove inner padding and border in Firefox 4+.
 */

button::-moz-focus-inner,
input::-moz-focus-inner {
  border: 0;
  padding: 0;
}

/**
 * Address Firefox 4+ setting `line-height` on `input` using `!important` in
 * the UA stylesheet.
 */

input {
  line-height: normal;
}

/**
 * It's recommended that you don't attempt to style these elements.
 * Firefox's implementation doesn't respect box-sizing, padding, or width.
 *
 * 1. Address box sizing set to `content-box` in IE 8/9/10.
 * 2. Remove excess padding in IE 8/9/10.
 */

input[type="checkbox"],
input[type="radio"] {
  box-sizing: border-box; /* 1 */
  padding: 0; /* 2 */
}

/**
 * Fix the cursor style for Chrome's increment/decrement buttons. For certain
 * `font-size` values of the `input`, it causes the cursor style of the
 * decrement button to change from `default` to `text`.
 */

input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
  height: auto;
}

/**
 * 1. Address `appearance` set to `searchfield` in Safari and Chrome.
 * 2. Address `box-sizing` set to `border-box` in Safari and Chrome
 *    (include `-moz` to future-proof).
 */

input[type="search"] {
  -webkit-appearance: textfield; /* 1 */
  -moz-box-sizing: content-box;
  -webkit-box-sizing: content-box; /* 2 */
  box-sizing: content-box;
}

/**
 * Remove inner padding and search cancel button in Safari and Chrome on OS X.
 * Safari (but not Chrome) clips the cancel button when the search input has
 * padding (and `textfield` appearance).
 */

input[type="search"]::-webkit-search-cancel-button,
input[type="search"]::-webkit-search-decoration {
  -webkit-appearance: none;
}

/**
 * Define consistent border, margin, and padding.
 */

fieldset {
  border: 1px solid #c0c0c0;
  margin: 0 2px;
  padding: 0.35em 0.625em 0.75em;
}

/**
 * 1. Correct `color` not being inherited in IE 8/9/10/11.
 * 2. Remove padding so people aren't caught out if they zero out fieldsets.
 */

legend {
  border: 0; /* 1 */
  padding: 0; /* 2 */
}

/**
 * Remove default vertical scrollbar in IE 8/9/10/11.
 */

textarea {
  overflow: auto;
}

/**
 * Don't inherit the `font-weight` (applied by a rule above).
 * NOTE: the default cannot safely be changed in Chrome and Safari on OS X.
 */

optgroup {
  font-weight: bold;
}

/* Tables
   ========================================================================== */

/**
 * Remove most spacing between table cells.
 */

table {
  border-collapse: collapse;
  border-spacing: 0;
}

td,
th {
  padding: 0;
}


.line0 sub, .line0 sup { line-height: 0; font-size: 75%;}

.normalize sub,
.normalize sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline;
}

.normalize sup {
  top: -0.5em;
}

.normalize sub {
  bottom: -0.25em;
}

<p class="line0">Hello <sub>there,</sub> How<sup>are you?</sup>
</p>

<p class="normalize">Hello <sub>there,</sub> How<sup>are you?</sup>
</p>