css

border

 #box{
    width: 100px;
    height: 100px;
    background: yellowgreen;
    box-shadow: 0px 0px 0px 10px #655;
    margin-left: 50px;
    margin-top: 50px;
}

#box{
    width: 100px;
    height: 100px;
    background: yellowgreen;
    box-shadow: 0px 0px 0px 10px #655,0px 0px 0px 15px deeppink inset;
    margin-left: 50px;
    margin-top: 50px;
}

outline方案

 #box{
    width: 100px;
    height: 100px;
    background: yellowgreen;
    border: 10px solid #655;
    outline: 5px solid deeppink;
}

background-position的扩展语法方案

 #box{
    width: 100px;
    height: 100px;
    background: url('../image/bg.png') no-repeat;
    border: 1px solid #ccc;
    background-position: right 20px bottom 10px;
}

边框内圆角

.something{
    width: 300px;
    background: #655;
    padding: .8em;
}
.something > div{
    background: tan;
    border-radius: .8em;
    padding: 1em;
}
<div class="something">
    <div>这是一个边框内圆角</div>
</div>

条纹背景

#box{
    width: 100px;
    height: 100px;
    background: linear-gradient(#fb3,#58a);
}

三条条纹

#box{
    width: 100px;
    height: 100px;
    background: linear-gradient(#fb3 33.3%,#58a 0,#58a 66.6%,yellowgreen 0);
    background-size: 100% 45px;
}

垂直条纹

#box{
    width: 100px;
    height: 100px;
    background: linear-gradient(to right, #fb3 50%, #58a 0);
    background-size: 30px 100%;
}

垂直条纹

网格

#box{
    width: 210px;
    height: 210px;
    border-right: 1px solid red;
    border-bottom: 1px solid red;
    background-image: 
    linear-gradient(red 1px,transparent 0),
    linear-gradient(90deg,red 1px,transparent 0);
    background-size: 30px 30px;
    ;
}

波点

#box{
    width: 200px;
    height: 200px;
    background: #655;
    background-image: radial-gradient(tan 30%,transparent 0);
    background-size: 30px 30px;
}

波点图2

#box{
    width: 200px;
    height: 200px;
    background-size: 30px 30px;
    background-image: radial-gradient(tan 30%,transparent 0),
    radial-gradient(red 30%,transparent 0);
    background-position: 0 0,15px 15px;
}

老式信封

div {
	padding: 1em;
	border: 1em solid transparent;
	background: linear-gradient(white, white) padding-box,
	            repeating-linear-gradient(-45deg, red 0, red 12.5%, transparent 0, transparent 25%, 
	              #58a 0, #58a 37.5%, transparent 0, transparent 50%) 0 / 6em 6em;
	
	max-width: 20em;
	font: 100%/1.6 Baskerville, Palatino, serif;
}

由边形

#button{
    padding: 8px 16px;
    display: inline-block;
    background: blueviolet;
    border-radius: 4px;
    text-decoration-line: none;
    color: #fff;
    transform: skew(-45deg);
}
#button > div{
    transform: skew(45deg);
}

#button{
    padding: 8px 16px;
    border-radius: 4px;
    color: #fff;
    position: relative;
    text-decoration-line: none;
}
#button::before{
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: 1;
    color: #fff;
    z-index: -1;
    background: greenyellow;
    transform: skewX(-46deg);
}

切角

#box{
    width: 200px;
    height: 200px;
    background: #58a;
    background: linear-gradient(-45deg,transparent 15px,#58a 0);
}

(https://imgtu.com/i/gKVI2V)

孤形切角

#box{
    background: #58a;
	background:	radial-gradient(circle at top left, transparent 15px, #58a 0) top left,
	            radial-gradient(circle at top right, transparent 15px, #58a 0) top right,
	            radial-gradient(circle at bottom right, transparent 15px, #58a 0) bottom right,
	            radial-gradient(circle at bottom left, transparent 15px, #58a 0) bottom left;
	background-size: 50% 50%;
	background-repeat: no-repeat;
	
	padding: 1em 1.2em;
	max-width: 12em;
	color: white;
	font: 130%/1.6 Baskerville, Palatino, serif;

}

(https://imgtu.com/i/gKZdLF)