Convert PostCSS to 'normal' CSS

I have a piece of code styled with PostCSS. Now I want to achieve the same effect in 'normal' CSS without using the PostCSS plugin. Is this possible and if yes, how?

This might sound like a dumb question, but I'm new to HTML/CSS

The code is here on Codepen: https://codepen.io/MightyFool/pen/jLKVqQ

Here the HTML:

<p>
  "Attractive things make people </a> <a class="link-1" href="#">feel good</a>, which in turn makes them think more</a> <a class="link-1" href="#">Creatively</a>. How does that make something easier to use? Simple, by making it easier for <a class="link-2">people</a> to find solutions to the problems they encounter." <strong>- Don Norman</strong>
</p>\

Here is the PostCSS CSS:

@use postcss-simple-vars;
@use postcss-nested;
@import 'https://fonts.googleapis.com/css?family=Open+Sans';

$underlineColor: #00458D;
$underlineHeight: 1px;

* {
  box-sizing: border-box;
}

html, body {
  font-family: 'Open Sans', sans-serif;
  width: 100%;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}

body {
  border: 8px solid $underlineColor;
}

a {
  cursor: pointer;
}

strong {
  margin-top: 16px;
  display: block;
  font-weight: 700;
}

p {
  padding: 24px;
  max-width: 760px;
  font-size: 16px;
  font-weight: 500;
  line-height: 24px;
}

.link-1 {
  position: relative;
  text-decoration: none;
  display: inline-block;
  color: #00458D;
  padding: 0 1px;
  transition: color ease 0.25s;

  &::after {
    content: '';
    position: absolute;
    z-index: -1;
    width: 100%;
    height: $underlineHeight;
    left: 0;
    bottom: 0;
    background-color: $underlineColor;
    transition: all ease 0.25s;
  }

  &:active {
    position: relative;
    text-decoration: none;
    display: inline-block;
    background-color: #D42E1C;
      }

  &:hover {
    color: white;

    &::after {
      height: 100%;
    }
  }
}

It has to be possible, since Browsers just understand pure CSS. If you're using a preprocessor like PostCSS (Or SASS, LESS, Stylus ...) you will need a compiler which translates the Language specific Style Declarations it into plain CSS. Since you mentioned CodePen, there is even a Dropdown menu on the top right of the CSS Window, where you can choose "View Compiled CSS", which will give you this result:

@import 'https://fonts.googleapis.com/css?family=Open+Sans';

* {
  box-sizing: border-box;
}

html, body {
  font-family: 'Open Sans', sans-serif;
  width: 100%;
  height: 100%;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
}

body {
  border: 8px solid #00458D;
}

a {
  cursor: pointer;
}

strong {
  margin-top: 16px;
  display: block;
  font-weight: 700;
}

p {
  padding: 24px;
  max-width: 760px;
  font-size: 16px;
  font-weight: 500;
  line-height: 24px;
}

.link-1 {
  position: relative;
  text-decoration: none;
  display: inline-block;
  color: #00458D;
  padding: 0 1px;
  -webkit-transition: color ease 0.25s;
  transition: color ease 0.25s;
}

.link-1::after {
  content: '';
  position: absolute;
  z-index: -1;
  width: 100%;
  height: 1px;
  left: 0;
  bottom: 0;
  background-color: #00458D;
  -webkit-transition: all ease 0.25s;
  transition: all ease 0.25s;
}

.link-1:active {
  position: relative;
  text-decoration: none;
  display: inline-block;
  background-color: #D42E1C;
}

.link-1:hover {
  color: white;
}

.link-1:hover::after {
  height: 100%;
}

You may want to read this Article to get a better understanding on how to use CSS Preprocessors, and what they do.