How can I achieve this angled slash (/) using CSS? It should be responsive and should also have IE 8 support?

How can I achieve this angled slash (/) using CSS? It should be responsive and should also have IE 8 support?

You can use an element and hide the unwanted parts with pseudo elements and borders (which are supported by IE8):
div{
position:relative;
width: 100px; height: 100px;
background: #000;
}
div:before, div:after{
content: '';
position: absolute;
}
div:before{
top: 0; left: 0;
border-left: 99px solid #fff;
border-bottom: 99px solid transparent;
}
div:after{
bottom: 0; right: 0;
border-right: 99px solid #fff;
border-top: 99px solid transparent;
}
<div></div>
You can control the slant by changing the border widths on the pseudo elements. For an explanation of how this works, see here: How do CSS triangles work?
Although this can't be implemented over an image or gradient because of the plain color triangles, It can be responsive.
If you want to use CSS for creating a content, You must use the CSS content property and you must apply it to CSS property with ::before selector. Here is the example
.number::before {content:"\2215"}
<span class="number">12</span>
The only difference is,you must use ::before with one semicolon for IE8 :before
IE8 Supports both the CSS :before Pseudo selector,and CSS content property.
For the content property,you need to find the specific "slash" that you want,in the Unicode table