My basic CSS resets

base.css

* {
  box-sizing: border-box;

  font-family: 'Helvetica Neue', Arial, sans-serif;
}

html,
body,
h1,
h2,
h3,
h4,
h5,
h6,
p,
ol,
ul {
  margin: 0;
  padding: 0;
}

html {
  color: black;
  color: hsl(0, 0%, 20%);
  background-color: white;
  background-color: hsl(0, 0%, 100%);

  font-size: 1rem;
  line-height: 1.6;
}

::selection {
  background-color: yellow;
  background-color: hsla(80, 100%, 50%, 50%);
}

h1,
h2,
h3,
h4,
h5,
h6 {
  line-height: 1.2;
  margin-top: 1em;
}

h1 {
  font-size: 2rem;
  margin-top: 2em;
}

h2 {
  font-size: 1.5rem;
  margin-top: 2em;
}

h3 {
  font-size: 1.25rem;
}

p,
ul,
ol,
pre {
  margin-top: 0.75em;
}

li {
  margin-top: 0.25em;
}

code {
  display: inline;

  font-size: 1.25em;
}

code, 
pre {
  background-color: lightgray;
  background-color: hsl(204, 15%, 94%);
  border-radius: 0.2em;
  padding: 0.3em 0.4em;

  font-family: monospace;
}

a {
  color: blue;
  color: hsl(210, 100%, 50%);
  text-decoration: none;
}

a:hover {
  /*border-bottom: 2px hsl(210, 100%, 55%) solid;*/
  text-decoration: underline
}

a:visited {
  color: magenta;
  color: hsl(210, 100%, 40%);
}

ul,
ol {
  list-style-position: outside;
  padding-left: 1.75em; 
}

ul > li,
ol > li {
  padding-left: 0.5em;
}

ul > li {
  list-style: disc;
}

hr {
  border: none;
  border-top: 1px solid lightgray;
  border-top: 1px solid hsl(0, 0%, 90%);
  margin: 50px 0;
}

input,
textarea,
select,
button,
fieldset {
  border: 1px solid lightgray;
  border: 1px solid hsl(0, 0%, 90%);
  border-radius: 2px;
}

button,
input,
select,
textarea {
  background: none;
  background-color: white;
  background-color: hsl(0, 0%, 100%);
  color: black;
  color: hsl(0, 0%, 20%);
  font-size: 1rem;
  line-height: 1.6;
  padding: 0.2em 0.5em;
}

::placeholder {
  color: lightgray;
  color: hsl(0, 0%, 70%);
}

select {
  display: inline-block;
  -moz-appearance: none;
  -webkit-appearance: none;
  appearance: none;

  background-image:url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' version='1.1' height='10px' width='10px'><polygon points='1,1 9,1 5,9' style='fill:hsl(0, 0%, 70%);' /></svg>");
  background-origin: border-box;
  background-repeat: no-repeat;
  background-position: center right 5px;
  padding-right: 1em;
}

button:hover,
input[type=button]:hover,
input[type=submit]:hover {
  background: lightgray;
  background: hsl(0, 0%, 90%);
}

button:active,
input[type=button]:active,
input[type=submit]:active {
  background: gray;
  background: hsl(0, 0%, 80%);
  border-color: gray;
  border-color: hsl(0, 0%, 80%);
}


img {
  max-width: 100%;
  height: auto;
}