This is what Bootstrap 4.0.0's Reboot looks like with all the Normalize styles removed. For educational purposes only, just use Reboot if you like these styles.

reboot-minus-normalize.css

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  font-family: sans-serif;
  -ms-overflow-style: scrollbar;
  -webkit-tap-highlight-color: transparent;
}

@-ms-viewport {
  width: device-width;
}

article, aside, dialog, figcaption, figure, footer, header, hgroup, main, nav, section {
  display: block;
}

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
  font-size: 1rem;
  font-weight: 400;
  line-height: 1.5;
  color: #212529;
  text-align: left;
  background-color: #fff;
}

[tabindex="-1"]:focus {
  outline: 0 !important;
}

h1, h2, h3, h4, h5, h6 {
  margin-top: 0;
  margin-bottom: 0.5rem;
}

p {
  margin-top: 0;
  margin-bottom: 1rem;
}

abbr[title],
abbr[data-original-title] {
  cursor: help;
  border-bottom: 0;
}

address {
  margin-bottom: 1rem;
  font-style: normal;
  line-height: inherit;
}

ol,
ul,
dl {
  margin-top: 0;
  margin-bottom: 1rem;
}

ol ol,
ul ul,
ol ul,
ul ol {
  margin-bottom: 0;
}

dt {
  font-weight: 700;
}

dd {
  margin-bottom: .5rem;
  margin-left: 0;
}

blockquote {
  margin: 0 0 1rem;
}

dfn {
  font-style: italic;
}

a {
  color: #007bff;
  text-decoration: none;
  -webkit-text-decoration-skip: objects;
}

a:hover {
  color: #0056b3;
  text-decoration: underline;
}

a:not([href]):not([tabindex]) {
  color: inherit;
  text-decoration: none;
}

a:not([href]):not([tabindex]):hover, a:not([href]):not([tabindex]):focus {
  color: inherit;
  text-decoration: none;
}

a:not([href]):not([tabindex]):focus {
  outline: 0;
}

pre {
  margin-top: 0;
  margin-bottom: 1rem;
  overflow: auto;
  -ms-overflow-style: scrollbar;
}

figure {
  margin: 0 0 1rem;
}

img {
  vertical-align: middle;
}

svg:not(:root) {
  overflow: hidden;
}

table {
  border-collapse: collapse;
}

caption {
  padding-top: 0.75rem;
  padding-bottom: 0.75rem;
  color: #6c757d;
  text-align: left;
  caption-side: bottom;
}

th {
  text-align: inherit;
}

label {
  display: inline-block;
  margin-bottom: .5rem;
}

button {
  border-radius: 0;
}

button:focus {
  outline: 1px dotted;
  outline: 5px auto -webkit-focus-ring-color;
}

input[type="radio"],
input[type="checkbox"] {
  box-sizing: border-box;
  padding: 0;
}

input[type="date"],
input[type="time"],
input[type="datetime-local"],
input[type="month"] {
  -webkit-appearance: listbox;
}

textarea {
  resize: vertical;
}

fieldset {
  min-width: 0;
  padding: 0;
  margin: 0;
  border: 0;
}

legend {
  display: block;
  width: 100%;
  margin-bottom: .5rem;
  font-size: 1.5rem;
  line-height: inherit;
}

output {
  display: inline-block;
}

summary {
  cursor: pointer;
}