你缺少了一个“重置CSS”，它可以覆盖浏览器的默认样式。
注意：您可能会遗漏其他不相关的填充/边距样式（例如输入元素之间的差距）

/* http://meyerweb.com/eric/tools/css/reset/ 
   v2.0 | 20110126
   License: none (public domain)
*/

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
    margin: 0;
    padding: 0;
    border: 0;
    font-size: 100%;
    font: inherit;
    vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure, 
footer, header, hgroup, menu, nav, section {
    display: block;
}
body {
    line-height: 1;
}
ol, ul {
    list-style: none;
}
blockquote, q {
    quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
    content: '';
    content: none;
}
table {
    border-collapse: collapse;
    border-spacing: 0;
}

/**/

.card {
  position: relative;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-direction: column;
  flex-direction: column;
  min-width: 0;
  word-wrap: break-word;
  background-color: #fff;
  background-clip: border-box;
  border: 1px solid rgba(0, 0, 0, 0.125);
  border-radius: 0.25rem;
}

.card-header:first-child {
  border-radius: calc(0.25rem - 1px) calc(0.25rem - 1px) 0 0;
}

.card-header {
  padding: 0.75rem 1.25rem;
  margin-bottom: 0;
  background-color: rgba(0, 0, 0, 0.03);
  border-bottom: 1px solid rgba(0, 0, 0, 0.125);
}

.card-body {
  -ms-flex: 1 1 auto;
  flex: 1 1 auto;
  min-height: 1px;
  padding: 1.25rem;
}

.form-group {
  margin-bottom: 1rem;
}

label {
  display: inline-block;
  margin-bottom: 0.5rem;
}

.form-control {
  display: block;
  width: 100%;
  height: calc(1.5em + 0.75rem + 2px);
  padding: 0.375rem 0.75rem;
  font-size: 1rem;
  font-weight: 400;
  line-height: 1.5;
  color: #495057;
  background-color: #fff;
  background-clip: padding-box;
  border: 1px solid #ced4da;
  border-radius: 0.25rem;
  transition: border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
}

.form-row {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
  flex-wrap: wrap;
  margin-right: -5px;
  margin-left: -5px;
}

.form-row > .col,
.form-row > [class*='col-'] {
  padding-right: 5px;
  padding-left: 5px;
}

.col {
  -ms-flex-preferred-size: 0;
  flex-basis: 0;
  -ms-flex-positive: 1;
  flex-grow: 1;
  max-width: 100%;
  position: relative;
  width: 100%;
  padding-right: 15px;
  padding-left: 15px;
}

button,
input {
  overflow: visible;
}

.text-right {
  text-align: right !important;
}

.btn-primary {
  color: #fff;
  background-color: #007bff;
  border-color: #007bff;
}

.btn {
  display: inline-block;
  font-weight: 400;
  color: #212529;
  text-align: center;
  vertical-align: middle;
  cursor: pointer;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  background-color: transparent;
  border: 1px solid transparent;
  padding: 0.375rem 0.75rem;
  font-size: 1rem;
  line-height: 1.5;
  border-radius: 0.25rem;
  transition: color 0.15s ease-in-out, background-color 0.15s ease-in-out,
    border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
}

.mr-1,
.mx-1 {
  margin-right: 0.25rem !important;
}

.btn-secondary {
  color: #fff;
  background-color: #6c757d;
  border-color: #6c757d;
}

.m-3 {
  margin: 1rem !important;
}

个字符