//btn源码
.btn {
 display: inline-block;
 padding: 6px 12px;
 margin-bottom: 0;
 font-size: 14px;
 font-weight: normal;
 line-height: 1.42857143;
 text-align: center;
 white-space: nowrap;
 vertical-align: middle;
 -ms-touch-action: manipulation;
  touch-action: manipulation;
 cursor: pointer;
 -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
   user-select: none;
 background-image: none;
 border: 1px solid transparent;
 border-radius: 4px;
}

//源码
.btn-lg,
.btn-group-lg > .btn {
 padding: 10px 16px;
 font-size: 18px;
 line-height: 1.3333333;
 border-radius: 6px;
}
.btn-sm,
.btn-group-sm > .btn {
 padding: 5px 10px;
 font-size: 12px;
 line-height: 1.5;
 border-radius: 3px;
}
.btn-xs,
.btn-group-xs > .btn {
 padding: 1px 5px;
 font-size: 12px;
 line-height: 1.5;
 border-radius: 3px;
}

//源码
.btn-block {
 display: block;
 width: 100%;
}
.btn-block + .btn-block {
 margin-top: 5px;
}
input[type="submit"].btn-block,
input[type="reset"].btn-block,
input[type="button"].btn-block {
 width: 100%;
}

<a class="btn btn-default" href="#">链接</a>
<button class="btn btn-default" type="submit">按钮</button>
<input class="btn btn-default" type="submit" value="输入框">