You can draw two rectangles and place them next to each other. And then rotate by 45 degrees. Modify the width/height/top/left parameters for any variation.

DEMO 1

DEMO 2 (With circle)

HTML

<span class="checkmark">
    <div class="checkmark_stem"></div>
    <div class="checkmark_kick"></div>
</span>

CSS

.checkmark {
    display:inline-block;
    width: 22px;
    height:22px;
    -ms-transform: rotate(45deg); /* IE 9 */
    -webkit-transform: rotate(45deg); /* Chrome, Safari, Opera */
    transform: rotate(45deg);
}

.checkmark_stem {
    position: absolute;
    width:3px;
    height:9px;
    background-color:#ccc;
    left:11px;
    top:6px;
}

.checkmark_kick {
    position: absolute;
    width:3px;
    height:3px;
    background-color:#ccc;
    left:8px;
    top:12px;
}

Here is another CSS solution. its take less line of code.

ul li:before
{
    content: '\2713';
    display: inline-block;
    color: red;
    padding: 0 6px 0 0;
}

ul li
{
    list-style-type: none;
    font-size: 1em;
}

<ul>
    <li>test1</li>
    <li>test</li>
</ul>

Here is the Demo link

Do some transforms with the letter L

.checkmark {
  font-family: arial;
  -ms-transform: scaleX(-1) rotate(-35deg); /* IE 9 */
  -webkit-transform: scaleX(-1) rotate(-35deg); /* Chrome, Safari, Opera */
  transform: scaleX(-1) rotate(-35deg);
}

<div class="checkmark">L</div>

only css, quite simple I find it:

.checkmark {
      display: inline-block;
      transform: rotate(45deg);
      height: 25px;
      width: 12px;
      margin-left: 60%; 
      border-bottom: 7px solid #78b13f;
      border-right: 7px solid #78b13f;
    }

<div class="checkmark"></div>

An additional solution, for when you only have one of the :before / :after psuedo-elements available, is described here: :after-Checkbox using borders

It basically uses the border-bottom and border-right properties to create the checkbox, and then rotates the mirrored L using transform

Example

li {
  position: relative; /* necessary for positioning the :after */
}

li.done {
  list-style: none; /* remove normal bullet for done items */
}

li.done:after {
  content: "";
  background-color: transparent;
  
  /* position the checkbox */
  position: absolute;
  left: -16px;
  top: 0px;

  /* setting the checkbox */
    /* short arm */
  width: 5px;
  border-bottom: 3px solid #4D7C2A;
    /* long arm */
  height: 11px;
  border-right: 3px solid #4D7C2A;
  
  /* rotate the mirrored L to make it a checkbox */
  transform: rotate(45deg);
    -o-transform: rotate(45deg);
    -ms-transform: rotate(45deg);
    -webkit-transform: rotate(45deg);
}

To do:
<ul>
  <li class="done">Great stuff</li>
  <li class="done">Easy stuff</li>
  <li>Difficult stuff</li>
</ul>

I've used something similar to BM2ilabs's answer in the past to style the tick in checkboxes. This technique uses only a single pseudo element so it preserves the semantic HTML and there is no reason for additional HTML elements.

label {
  cursor: pointer;
}

input[type="checkbox"] {
  position: relative;
  top: 2px;
  box-sizing: content-box;
  width: 14px;
  height: 14px;
  margin: 0 5px 0 0;
  cursor: pointer;
  -webkit-appearance: none;
  border-radius: 2px;
  background-color: #fff;
  border: 1px solid #b7b7b7;
}

input[type="checkbox"]:before {
  content: '';
  display: block;
}

input[type="checkbox"]:checked:before {
  width: 4px;
  height: 9px;
  margin: 0px 4px;
  border-bottom: 2px solid #115c80;
  border-right: 2px solid #115c80;
  transform: rotate(45deg);
}

<label>
  <input type="checkbox" name="check-1" value="Label">Label
</label>

Try this // html example

<span>&#10003;</span>

// css example

span {
  content: "\2713";
}

i like this way because you don't need to create two components just one.

.checkmark:after {
    opacity: 1;
    height: 4em;
    width: 2em;
    -webkit-transform-origin: left top;
    transform-origin: left top;
    border-right: 2px solid #5cb85c;
    border-top: 2px solid #5cb85c;
    content: '';
    left: 2em;
    top: 4em;
    position: absolute;
}

Animation from Scott Galloway Pen

I suggest to use a tick symbol not draw it. Or use webfonts which are free for example: fontello[dot]com You can than replace the tick symbol with a web font glyph.

Lists

ul {padding: 0;}
li {list-style: none}
li:before {
  display:inline-block;
  vertical-align: top;
  line-height: 1em;
  width: 1em;
  height:1em;
  margin-right: 0.3em;
  text-align: center;
  content: '✔';
  color: #999;
}

See here:

Checkboxes

You even have web fonts with tick symbol glyphs and CSS 3 animations. For IE8 you would need to apply a polyfill since it does not understand :checked.

input[type="checkbox"] {
  clip: rect(1px, 1px, 1px, 1px);
  left: -9999px;
  position: absolute !important;
}
label:before,
input[type="checkbox"]:checked + label:before {
  content:'';
  display:inline-block;
  vertical-align: top;
  line-height: 1em;
  border: 1px solid #999;
  border-radius: 0.3em;
  width: 1em;
  height:1em;
  margin-right: 0.3em;
  text-align: center;
}
input[type="checkbox"]:checked + label:before {
  content: '✔';
  color: green;
}

See the JS fiddle:

li:before {
    content: '';
    height: 5px;
    background-color: green;
    position: relative;
    display: block;
    top: 50%;
    left: 50%;
    width: 9px;
    margin-left: -15px;
    -ms-transform: rotate(45deg);
    -webkit-transform: rotate(45deg);
    transform: rotate(45deg);
}
li:after {
    content: '';
    height: 5px;
    background-color: green;
    position: relative;
    display: block;
    top: 50%;
    left: 50%;
    width: 20px;
    margin-left: -11px;
    margin-top: -6px;
    -ms-transform: rotate(-45deg);
    -webkit-transform: rotate(-45deg);
    transform: rotate(-45deg);
}

After some changing to above Henry's answer, I got a tick with in a circle, I came here looking for that, so adding my code here.

.snackbar_circle {
  background-color: #f0f0f0;
  border-radius: 13px;
  padding: 0 5px;
}

.checkmark {
  font-family: arial;
  font-weight: bold;
  -ms-transform: scaleX(-1) rotate(-35deg);
  -webkit-transform: scaleX(-1) rotate(-35deg);
  transform: scaleX(-1) rotate(-35deg);
  color: #63BA3D;
  display: inline-block;
}

<span class="snackbar_circle">
   <span class="checkmark">L</span>
</span>

You might want to try fontawesome.io

It has great collection of icons. For you <i class="fa fa-check" aria-hidden="true"></i> should work. There are many check icons in this too. Hope it helps.

Also, using the awesome font, you can use the following tag. Simple and beautiful

With the possibility of changing the size and color and other features in CSS

See result here

This is my variant for making 'checked' buttons

HTML

<button class="unchecked" type="submit" onclick="clickMe(this); return false;" value="something"></button>

JavaScript

function clickMe(data) {
  data.classList.add("checked");
}

CSS

.unchecked::before { content: "C"; }
.unchecked { border-radius: 50%; outline: none; }
.checked::before { content: "L"; }
.checked { background-color: green; transform: rotate(45deg) scaleX(-1); }

We can use CSS pseudo-element to make the checkmark/tick sign. Suppose, we have a span tag in our HTML and we want to place out checkmark before the span. We can simply do this:

<span class="check_text">Some Text</span>

Now, we can add the CSS like this:

span.check_text:before {
    position: absolute;
    height: 15px;
    width: 5px;
    border-bottom: 3px solid red;
    border-right: 3px solid red;
    content: "";
    transform: rotate(45deg);
    left: -25px;
    top: 2px;
}

This is simple css for Sign Mark.

ul li:after{opacity: 1;content: '\2713';right: 20px;position: absolute;font-size: 20px;font-weight: bold;}