Since you are using a css file, you will need to convert the scss to css.

Here's a demo:

.checkmark__circle {
  stroke-dasharray: 166;
  stroke-dashoffset: 166;
  stroke-width: 2;
  stroke-miterlimit: 10;
  stroke: #7ac142;
  fill: none;
  animation: stroke 0.6s cubic-bezier(0.65, 0, 0.45, 1) forwards;
}

.checkmark {
  width: 56px;
  height: 56px;
  border-radius: 50%;
  display: block;
  stroke-width: 2;
  stroke: #fff;
  stroke-miterlimit: 10;
  margin: 10% auto;
  box-shadow: inset 0px 0px 0px #7ac142;
  animation: fill .4s ease-in-out .4s forwards, scale .3s ease-in-out .9s both;
}

.checkmark__check {
  transform-origin: 50% 50%;
  stroke-dasharray: 48;
  stroke-dashoffset: 48;
  animation: stroke 0.3s cubic-bezier(0.65, 0, 0.45, 1) 0.8s forwards;
}

@keyframes stroke {
  100% {
    stroke-dashoffset: 0;
  }
}
@keyframes scale {
  0%, 100% {
    transform: none;
  }
  50% {
    transform: scale3d(1.1, 1.1, 1);
  }
}
@keyframes fill {
  100% {
    box-shadow: inset 0px 0px 0px 30px #7ac142;
  }
}

<svg class="checkmark" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 52 52">
  <circle class="checkmark__circle" cx="26" cy="26" r="25" fill="none"/>
  <path class="checkmark__check" fill="none" d="M14.1 27.2l7.1 7.2 16.7-16.8"/>
</svg>

SASS & SCSS Info:

Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.

Sass has two syntaxes. The new main syntax (as of Sass 3) is known as “SCSS” (for “Sassy CSS”), and is a superset of CSS3’s syntax. This means that every valid CSS3 stylesheet is valid SCSS as well. SCSS files use the extension .scss.

The second, older syntax is known as the indented syntax (or just “Sass”). Inspired by Haml’s terseness, it’s intended for people who prefer conciseness over similarity to CSS. Instead of brackets and semicolons, it uses the indentation of lines to specify blocks. Although no longer the primary syntax, the indented syntax will continue to be supported. Files in the indented syntax use the extension .sass.

Well, this is my approach using <canvas> and JavaScript.

Demo on Fiddle -----> Square Corners | Round Corners

(Note: To change the animation speed increment or decrement the variable animationSpeed. Lower number yeilds Higher speed)

var start = 100;
var mid = 145;
var end = 250;
var width = 20;
var leftX = start;
var leftY = start;
var rightX = mid - (width / 2.7);
var rightY = mid + (width / 2.7);
var animationSpeed = 20;

var ctx = document.getElementsByTagName('canvas')[0].getContext('2d');
ctx.lineWidth = width;
ctx.strokeStyle = 'rgba(0, 150, 0, 1)';

for (i = start; i < mid; i++) {
  var drawLeft = window.setTimeout(function() {
    ctx.beginPath();
    ctx.moveTo(start, start);
    ctx.lineTo(leftX, leftY);
    ctx.stroke();
    leftX++;
    leftY++;
  }, 1 + (i * animationSpeed) / 3);
}

for (i = mid; i < end; i++) {
  var drawRight = window.setTimeout(function() {
    ctx.beginPath();
    ctx.moveTo(leftX, leftY);
    ctx.lineTo(rightX, rightY);
    ctx.stroke();
    rightX++;
    rightY--;
  }, 1 + (i * animationSpeed) / 3);
}

<canvas height="160"></canvas>

You could also do this using svg and CSS animation.

1. Square Corners:

#check {
  fill: none;
  stroke: green;
  stroke-width: 20;
  stroke-dasharray: 180;
  stroke-dashoffset: 180;
  -webkit-animation: draw 2s infinite ease;
  animation: draw 2s infinite ease;
}
-webkit-@keyframes draw {
  to {
    stroke-dashoffset: 0;
  }
}
@keyframes draw {
  to {
    stroke-dashoffset: 0;
  }
}

<svg width="150" height="150">
  <path id="check" d="M10,30 l30,50 l95,-70" />
</svg>

2. Round Corners:

#check {
  fill: none;
  stroke: green;
  stroke-width: 20;
  stroke-linecap: round;
  stroke-dasharray: 180;
  stroke-dashoffset: 180;
  animation: draw 2s infinite ease;
}
@keyframes draw {
  to {
    stroke-dashoffset: 0;
  }
}

<svg width="150" height="150">
  <path id="check" d="M10,50 l25,40 l95,-70" />
</svg>