校正<sup> & <sub>线高

Question

在Normalize.css中，位置相对于<sup> & <sub>修复直线高度问题真的是必要的吗？

/**
 * 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;
}

使用浏览器默认的vertical-align: sup/sub有什么问题

或者换句话说，为什么不直接声明sub, sup { line-height: 0; }呢？

Answer 1

让我们看看normalize.css背后的想法。来自normalize.css Github页面：

Normalize.css使浏览器更加一致地呈现所有元素，并且符合现代标准。它只针对需要规范化的样式。

从这个声明中获取的主要关键字是“一致的”。

在这些截图中，顶部有line-height: 0和font-size: 75%用于sub和sup。底线具有规范化的CSS，并且在浏览器之间定位更加一致。

IE6

​

​

IE8

​

​

IE11

​

​

Chrome --在现代浏览器中，这种差异并不大。

​

​

差别很小，但是标准化的CSS给出了浏览器之间更一致的外观，包括IE6。使用position: relative实现了这种一致性。

这就是我用来截图的东西

也是用jsbin链接插入到jsBin例子。中的netrenderer.com IE屏幕截图

​

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>

​

Answer 2

不，如果使用sub, sup { line-height: 0; }，则不需要相对定位来纠正直线高度问题。另一方面，设置vertical-align: baseline就足以解决这个问题。因此，从这个意义上说，代码包含了两个修复程序，其中一个就足够了。然而，它显然也试图修复其他东西。

行高问题是，默认情况下，浏览器使用sup和sub来实现vertical-alignment，这使得线条的高度更大：

​

<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.

​

代码还试图通过使用相对位置而不是vertical-align，使浏览器之间的呈现更加相似。sub和super值对vertical-align的意义与浏览器有关，即下标和上标的垂直位置不同。原则上，这种变化可以通过使用em单元将属性设置为已知值来解决，但是浏览器被观察到错误地实现了这些设置。相对定位比较一致。

代码还试图通过设置字体缩减因子(否则取决于浏览器)来使呈现统一。在这里，代码实际上失败了，因为它实际上引入了不同浏览器之间的变化。原因是IE不正确地为sup和sub实现百分比字体大小。对于典型的基本字体大小为16 IE，75%为12 IE，而在IE上则为10 IE(尚不清楚原因)。

结论是，如果您希望在浏览器之间得到尽可能类似的上标和下标，则根本不使用sub和sup。相反，您可以将span与class属性结合使用，并定义所需的垂直位置和字体大小。