简单图形
矩形
div{
width: 100px;
height: 100px;
background-color: red;
}

div{
width: 100px;
height: 100px;
background-color: red;
border-radius: 50%;
}

椭圆
【整个椭圆】
div{
width: 100px;
height: 50px;
background-color: red;
border-radius: 50%;
}

div{
width: 100px;
height: 100px;
background:red;
border-radius:50% /100% 100% 0 0 ;
}

div{
width: 100px;
height: 100px;
background:red;
border-radius:100% 0 0 0 ;
}

三角形
【直角三角形】
div{
width: 0;
height: 0;
border: 50px solid transparent;
border-bottom-color: red;
}

【正三角形】
div{
width: 0;
height: 0;
border: 50px solid transparent;
border-width: 86.6px 50px;
border-bottom-color: red;
}

平行四边形
【基本图形】
div{
margin-left: 50px;
width: 100px;
height: 100px;
background-color: red;
transform: skew(30deg);
}

【改进版本】
上面的代码中,不仅形状发生了变形,内容也发生了变形。有两种方法改进
1、元素嵌套
<style>
.outer{
margin-left:20px;
width: 100px;
height: 100px;
background:red;
transform:skewX(-30deg);
}
.inner{
transform:skewX(30deg);
}
</style>
<div class="outer">
<div class="inner">小浩IT</div>
</div>

2、伪元素
把所有样式都应用到伪元素上,然后再对伪元素进行变形
div{
position:relative;
margin-left:20px;
width: 100px;
height: 100px;
text-indent: 30px;
}
div::before{
content:'';
position:absolute;
top: 0;right: 0;left: 0;bottom: 0;
transform:skewX(-30deg);
background:red;
z-index:-1;
}

梯形
【基本版本】
div{
width: 50px;
border: 50px solid transparent;
border-bottom-color: red;
}

【增强版本】
上面的代码虽然简单且巧妙,但无法在图形内容填充文本。下面是比较复杂的增强版本
div{
height: 100px;
width: 200px;
line-height: 100px;
position:relative;
display:inline-block;
text-align: center;
color:white;
padding: .5em 1em .35em;
}
div:before{
content:'';
position:absolute;
top: 0;right: 0;bottom: 0;left: 0;
z-index:-1;
background:#58a;
transform:scaleY(1.3) perspective(.5em) rotateX(5deg);
transform-origin: bottom;
}

【梯形选项卡】
<style>
a{
width: 120px;
text-align: center;
text-decoration: none;
color:inherit;
font-size: 20px
}
nav > a{
position:relative;
display:inline-block;
padding:.3em .1em 0;
}
nav > a:before{
content:'';
position:absolute;
top: 0;right: 0;left: 0;bottom: 0;
z-index:-1;
background:#ccc;
border:1px solid black;
border-radius: .5em .5em 0 0 ;
box-shadow: 0 .15em white inset;
transform:scaleY(1.3) perspective(.5em) rotateX(5deg);
transform-origin: bottom;
}
</style>
<nav>
<a href="#">HTML</a>
<a href="#">CSS</a>
<a href="#">Javascript</a>
</nav>

复杂图形
多角星
【五角星】
一个大三角形底部掏掉一个小三角形;两个前面的相同图形叠压
经计算,五角星的夹角为36度,若小三角形的侧边为a,则大三角形的侧边为2a(1+sin18),两个三角形共同的底边为2a(sin54)
<div class="box">
<div class="in"></div>
<div class="in"></div>
</div>
.box{
position: relative;
}
.in{
margin-left: 100px;
position: relative;
width: 0px;
border: 10px solid transparent;
border-width: 249px 81px;
border-bottom-color: red;
position: absolute;
}
.in:after{
content: "";
position:absolute;
border: 10px solid transparent;
border-width: 59px 81px;
border-bottom-color: white;
top: 133px;
left: -81px;
}
.in:last-child{
top: 100px;
left: -134px;
transform: rotate(-73deg);
}

【六角星】
两个三角形叠压
div{
position: relative;
width: 0;
border: 50px solid transparent;
border-width: 50px 43.4px;
border-bottom-color: red;
}
div:after{
position: absolute;
content:"";
width: 0;
border: 50px solid transparent;
border-width: 50px 43.4px;
border-top-color: red;
top: 16px;
left: -42px;
}

多边形
【六边形】
两个梯形拼接
div{
position: relative;
width: 50px;
border: 50px solid transparent;
border-bottom-color: red;
}
div:after{
position: absolute;
content:"";
width: 50px;
border: 50px solid transparent;
border-top-color: red;
top:50px;
left: -50px;
}

【八边形】
.outer{
width:100px;
height: 100px;
transform:rotate(45deg);
overflow: hidden;
}
.inner{
width:100%;
height: 100%;
transform:rotate(-45deg);
background:red;
}

【菱形】
.outer{
margin:50px 0 0 50px;
width:100px;
height: 100px;
transform:rotate(45deg);
overflow: hidden;
}
.inner{
width:100%;
height: 100%;
transform:rotate(-45deg) scale(1.42);
background:red;
}

桃心
创建一个方形div,分别用css控制div的两个伪元素平移到正方形相邻两边,圆形与边中点重合。最后将总的div旋转45度
div{
display:inline-block;
margin:50px;
height: 100px;
width: 100px;
background-color: red;
transform: rotate(-45deg);
}
div:before,div:after{
display:block;
content:"";
width: 100px;
height: 100px;
background-color: red;
border-radius: 50%;
margin-top:-50%;
}
div:after{
margin-left:50%;
}
<div></div>

切角效果
【单角】
background: linear-gradient(-45deg,transparent 5%,#58a 0);

【双角】
background: linear-gradient(-45deg,transparent 5%,#58a 0) right,
linear-gradient(45deg,transparent 5%,#58a 0) left;
background-size:50% 100%;
background-repeat:no-repeat;

【四角】
background: linear-gradient(-135deg,transparent 5%,#58a 0) top right,
linear-gradient(135deg,transparent 5%,#58a 0) top left,
linear-gradient(-45deg,transparent 5%,#58a 0) bottom right,
linear-gradient(45deg,transparent 5%,#58a 0) bottom left;
background-size:50% 50%;
background-repeat:no-repeat;

【弧形切角】
background: radial-gradient(circle at top right,transparent 5%,#58a 0) top right,
radial-gradient(circle at top left,transparent 5%,#58a 0) top left,
radial-gradient(circle at bottom right,transparent 5%,#58a 0) bottom right,
radial-gradient(circle at bottom left,transparent 5%,#58a 0) bottom left;
background-size:50% 50%;
background-repeat:no-repeat;

折角效果
background:linear-gradient(to left bottom,transparent 50%,rgba(0,0,0,0.4) 0) no-repeat 100% 0 /2em 2em, linear-gradient(-135deg,transparent 1.414em ,#58a 0);

饼图效果
饼图在网页中的运用极为普遍,比如简单的统计图表、进度指示器、定时器等
【静态效果】
<style>
@keyframes spin{
to{transform: rotate(.5turn);}
}
@keyframes bg{50%{background:#655;}}
.pie{
width: 100px;line-height: 100px;
position:relative;
background-color: yellowgreen;
border-radius: 50%;
color:transparent;
text-align:center;
background-image: linear-gradient(to right, transparent 50%,#655 0);
}
.pie:before{
content:'';
position:absolute;
top: 0;left: 50%;
width: 50%;height: 100%;
border-radius: 0 100% 100% 0 /50%;
background-color: inherit;
transform-origin: left;
animation: spin 50s linear infinite,bg 100s step-end infinite;
animation-play-state:paused;
animation-delay:inherit;
}
</style>
<div class="pie" style="animation-delay:-20s">20%</div>
<div class="pie" style="animation-delay:-90s">90%</div>

【动态效果】
<style>
@keyframes spin{
to{transform: rotate(.5turn);}
}
@keyframes bg{50%{background:#655;}}
.pie{
width: 100px;height: 100px;
position:relative;
background-color: yellowgreen;
border-radius: 50%;
background-image: linear-gradient(to right, transparent 50%,#655 0);
}
.pie:before{
content:'';
position:absolute;
top: 0;left: 50%;
width: 50%;height: 100%;
border-radius: 0 100% 100% 0 /50%;
background-color: inherit;
transform-origin: left;
animation: spin 3s linear infinite,bg 6s step-end infinite;
animation-play-state: paused;
}
.pie:hover:before{
animation-play-state: running;
}
</style>
<div class="pie"></div>
鼠标移入时,饼图发生移动

【SVG实现】
让圆形的周长接近于100,方便计算。r = 100/(2*PI) = 16
<style>
@keyframes fillup{to{stroke-dasharray:100 100;}}
svg{
width: 100px;
height: 100px;
transform: rotate(-90deg);
background: yellowgreen;
border-radius: 50%;
}
circle{
fill:yellowgreen;
stroke:#655;
stroke-width:32;
stroke-dasharray: 0 100;
animation: fillup 5s linear infinite;
}
</style>
<svg viewbox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
<circle r=16 cx=16 cy=16></circle>
</svg>

最后
除了使用CSS画图之后,实现各种形状更简单的方法是使用 clip-path路径裁剪样式
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