Fix <sup> & <sub> line-height

Question

In Normalize.css, is the relationship set for <sup> and <sub> to fix the row height problem?

 /** * 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 happened with the default browser vertical-align: sup/sub ?

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

Answer 1

Let's take a look at the idea of ​​normalize.css. From the normalize.css Github page:

Normalize.css makes browsers more consistently display all elements and in accordance with modern standards. It is precisely aimed only at styles that need normalization.

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

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

IE6

IE8

IE11

Chrome - Differences in modern browsers are not so big.

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

This is what I used for screenshots

Just like jsBin example. IE screenshots taken with 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 recommended that you don't attempt to style these elements. * Firefox 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 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> 

Answer 2

No, relative positioning is not required to fix the line height problem if you use sub, sup { line-height: 0; } sub, sup { line-height: 0; } . On the other hand, setting vertical-align: baseline enough to solve this problem. Thus, in this sense, the code contains two corrections when one of them is enough. However, apparently, he is trying to fix something else.

The problem with line height is that, by default, browsers implement sup and sub using vertical-alignment , which makes the line height larger:

 <p style="width: 10em"> This is an example of text that contains a superscript written using the HTML sup element: m<sup>2</sup>. The superscript causes uneven line spacing, due to the way browsers set lines. 

The code also tries to make rendering more similar in browsers using relative positioning rather than vertical-align . The value of the sub and super values ​​for vertical-align is browser dependent, i.e. the vertical position of the indices and add-ons is different. In principle, this change could be fixed by setting the property to a known value using the em block, but observers observed that such settings were made with an error. Relative positioning is more consistent.

The code is also trying to make the rendering uniform by setting the font reduction factor, which otherwise depends on the browser. Here, the code really fails, as it actually introduces changes in browsers. The reason is because IE does not correctly execute percentage font sizes for sup and sub . For a typical base font size of 16px, 75% gives 12px, but in IE it gives 10px (it’s not clear why).

The conclusion is that if you want to get superscript indices and indexes as similarly as possible in browsers, you don't use sub and sup . Instead, you use a span with class attributes and determine the desired vertical position and font size.