当你在浏览器控制台查看对应元素的样式的时候，会发现一些样式标有(user agent stylesheet),同时不能修改，那就表明该样式是浏览器自身对元素的设置样式，即浏览器默认样式表；

当然不同浏览器之间的默认样式会有出入，同时相同浏览器的不同版本之间也会有细微差别！！！

这里附上PC和APP端的相应reset.css样式表

reset.css  (PC端重置样式)

@charset "utf-8";
 
/* CSS Document */
* { color:#444; }
body { font:12px/1.5 "微软雅黑",Arial,Tahoma, Helvetica,b8bf53, sans-serif; }
html,body,div,h1,h2,h3,h4,h5,h6,hr,p,blockquote,dl,dt,dd,ul,ol,li,pre,form,fieldset,object,code,em,span,var,legend,button,input,textarea,th,td,a,img,header,footer,nav,aside,audio,datalist,section { margin:0;padding:0;border:0;outline:0; }/*清除内外边距*/
h1,h2,h3,h4,h5,h6 { font-weight:normal;font-size:100%; }/*设置默认字体*/
:focus { outline:0; }
ul,ol,ul li,ol li { list-style: none; }/*重置列表*/
address,caption,cite,em,code,dfn,th,var { font-style:normal;font-weight:normal; }
form label { cursor:pointer; }
input,button,select,textarea { font-family:inherit;font-size:100%;outline:none; }
textarea { resize:none }
input { vertical-align:middle; }
img { border:0; }/*重置图片元素*/
table { border-collapse:collapse;border-spacing: 0; }/*重置表格*/
.l { float:left; }
.r { float:right; }
button,input[type="reset"],input[type="button"],input[type="submit"] { line-height:normal !important; }
a { text-decoration:none;}
a { color:#666;}
a:hover { text-decoration:underline; }
input { _filter:chroma(color=#000000); }
/*--清除浮动
------------------------------------------------------------------------------------------*/
.clearfix:after { clear:both;content:".";display:block;font-size:0;height:0;visibility:hidden; }
.clearfix:after { _zoom:1; }
.overflow { overflow:hidden; _zoom:1; }
.clear { clear:both; height:0;font-size:0; overflow:hidden; }

 normalize.css 

Normalize.css 使浏览器呈现所有 HTML 元素更加一致，并且符合现代 web 标准。
Normalize.css 只作用于需要规范化的样式。

/*! normalize.css v8.0.1 | MIT License | github.com/necolas/normalize.css */
/* Document
   ========================================================================== */
/**
 * 1. Correct the line height in all browsers.
 * 2. Prevent adjustments of font size after orientation changes in iOS.
 */
html {
  line-height: 1.15; /* 1 */
  -webkit-text-size-adjust: 100%; /* 2 */
}
/* Sections
   ========================================================================== */
/**
 * Remove the margin in all browsers.
 */
body {
  margin: 0;
}
/**
 * Render the `main` element consistently in IE.
 */
main {
  display: block;
}
/**
 * Correct the font size and margin on `h1` elements within `section` and
 * `article` contexts in Chrome, Firefox, and Safari.
 */
h1 {
  font-size: 2em;
  margin: 0.67em 0;
}
/* Grouping content
   ========================================================================== */
/**
 * 1. Add the correct box sizing in Firefox.
 * 2. Show the overflow in Edge and IE.
 */
hr {
  box-sizing: content-box; /* 1 */
  height: 0; /* 1 */
  overflow: visible; /* 2 */
}
/**
 * 1. Correct the inheritance and scaling of font size in all browsers.
 * 2. Correct the odd `em` font sizing in all browsers.
 */
pre {
  font-family: monospace, monospace; /* 1 */
  font-size: 1em; /* 2 */
}
/* Text-level semantics
   ========================================================================== */
/**
 * Remove the gray background on active links in IE 10.
 */
a {
  background-color: transparent;
}
/**
 * 1. Remove the bottom border in Chrome 57-
 * 2. Add the correct text decoration in Chrome, Edge, IE, Opera, and Safari.
 */
abbr[title] {
  border-bottom: none; /* 1 */
  text-decoration: underline; /* 2 */
  text-decoration: underline dotted; /* 2 */
}
/**
 * Add the correct font weight in Chrome, Edge, and Safari.
 */
b,
strong {
  font-weight: bolder;
}
/**
 * 1. Correct the inheritance and scaling of font size in all browsers.
 * 2. Correct the odd `em` font sizing in all browsers.
 */
code,
kbd,
samp {
  font-family: monospace, monospace; /* 1 */
  font-size: 1em; /* 2 */
}
/**
 * Add the correct font size in all browsers.
 */
small {
  font-size: 80%;
}
/**
 * Prevent `sub` and `sup` elements from affecting the line height in
 * all browsers.
 */
sub,
sup {
  font-size: 75%;
  line-height: 0;
  position: relative;
  vertical-align: baseline;
}
sub {
  bottom: -0.25em;
}
sup {
  top: -0.5em;
}
/* Embedded content
   ========================================================================== */
/**
 * Remove the border on images inside links in IE 10.
 */
img {
  border-style: none;
}
/* Forms
   ========================================================================== */
/**
 * 1. Change the font styles in all browsers.
 * 2. Remove the margin in Firefox and Safari.
 */
button,
input,
optgroup,
select,
textarea {
  font-family: inherit; /* 1 */
  font-size: 100%; /* 1 */
  line-height: 1.15; /* 1 */
  margin: 0; /* 2 */
}
/**
 * Show the overflow in IE.
 * 1. Show the overflow in Edge.
 */
button,
input { /* 1 */
  overflow: visible;
}
/**
 * Remove the inheritance of text transform in Edge, Firefox, and IE.
 * 1. Remove the inheritance of text transform in Firefox.
 */
button,
select { /* 1 */
  text-transform: none;
}
/**
 * Correct the inability to style clickable types in iOS and Safari.
 */
button,
[type="button"],
[type="reset"],
[type="submit"] {
  -webkit-appearance: button;
}
/**
 * Remove the inner border and padding in Firefox.
 */
button::-moz-focus-inner,
[type="button"]::-moz-focus-inner,
[type="reset"]::-moz-focus-inner,
[type="submit"]::-moz-focus-inner {
  border-style: none;
  padding: 0;
}
/**
 * Restore the focus styles unset by the previous rule.
 */
button:-moz-focusring,
[type="button"]:-moz-focusring,
[type="reset"]:-moz-focusring,
[type="submit"]:-moz-focusring {
  outline: 1px dotted ButtonText;
}
/**
 * Correct the padding in Firefox.
 */
fieldset {
  padding: 0.35em 0.75em 0.625em;
}
/**
 * 1. Correct the text wrapping in Edge and IE.
 * 2. Correct the color inheritance from `fieldset` elements in IE.
 * 3. Remove the padding so developers are not caught out when they zero out
 *    `fieldset` elements in all browsers.
 */
legend {
  box-sizing: border-box; /* 1 */
  color: inherit; /* 2 */
  display: table; /* 1 */
  max-width: 100%; /* 1 */
  padding: 0; /* 3 */
  white-space: normal; /* 1 */
}
/**
 * Add the correct vertical alignment in Chrome, Firefox, and Opera.
 */
progress {
  vertical-align: baseline;
}
/**
 * Remove the default vertical scrollbar in IE 10+.
 */
textarea {
  overflow: auto;
}
/**
 * 1. Add the correct box sizing in IE 10.
 * 2. Remove the padding in IE 10.
 */
[type="checkbox"],
[type="radio"] {
  box-sizing: border-box; /* 1 */
  padding: 0; /* 2 */
}
/**
 * Correct the cursor style of increment and decrement buttons in Chrome.
 */
[type="number"]::-webkit-inner-spin-button,
[type="number"]::-webkit-outer-spin-button {
  height: auto;
}
/**
 * 1. Correct the odd appearance in Chrome and Safari.
 * 2. Correct the outline style in Safari.
 */
[type="search"] {
  -webkit-appearance: textfield; /* 1 */
  outline-offset: -2px; /* 2 */
}
/**
 * Remove the inner padding in Chrome and Safari on macOS.
 */
[type="search"]::-webkit-search-decoration {
  -webkit-appearance: none;
}
/**
 * 1. Correct the inability to style clickable types in iOS and Safari.
 * 2. Change font properties to `inherit` in Safari.
 */
::-webkit-file-upload-button {
  -webkit-appearance: button; /* 1 */
  font: inherit; /* 2 */
}
/* Interactive
   ========================================================================== */
/*
 * Add the correct display in Edge, IE 10+, and Firefox.
 */
details {
  display: block;
}
/*
 * Add the correct display in all browsers.
 */
summary {
  display: list-item;
}
/* Misc
   ========================================================================== */
/**
 * Add the correct display in IE 10+.
 */
template {
  display: none;
}
/**
 * Add the correct display in IE 10.
 */
[hidden] {
  display: none;
}

APP端初始化样式表

@charset "utf-8";
/* @group Public */
body,div,ul,ol,dl,li,dt,dd,h1,h2,h3,h4,p,form,iframe,input,textarea,a,span,em,strong,img,html,nav,header,article,button,footer,var {padding:0;margin:0;}
body { font:12px/1.2rem "Microsoft YaHei",tahoma,arial,sans-serif;min-width:320px;position:relative; }
form,input {background:none;border:none;}
ul,dl,ol {list-style-type:none;}
h1, h2, h3, h4, h5 { font:12px/1.2rem "Microsoft YaHei",arial,tahoma; }
a { text-decoration:none; }
a:hover,a:focus { outline:none; }
table { border-collapse:collapse;border-spacing:0; }
img { border:none; }
strong,b { font-weight:normal; }
em,i,var { font-style:normal; }
p { text-indent:0; }
.clear { clear:both;height:0;line-height:0;overflow:hidden;width:0; }
.clearfix:after { clear:both;content:".";display:block;font-size:0;height:0;visibility:hidden; }

一般推荐使用normalize.css，不会初始化掉基础的元素样式！！！