CSS无法将Div中的文本居中

masthead {
  top: 32px;
  padding-right: 0px;
  background: #000;
  padding-left: 0;
  max-height: 100px;
  border-bottom: none;
  position: fixed;
  z-index: 3;
  width: 100%;
  min-height: 73px;
  color: #000;
  display: block;
  box-sizing: inherit;
}

.sidebar-toggle {
  float: left;
  border: 3px solid grey;
  border-right: none;
  height: 82px;
  margin-right: 0;
  width: 5%;
  overflow: hidden;
  padding: 0;
  color: #000;
  text-align: center;
}

button {
  border-radius: 0;
  transition: all 125ms ease-out;
  cursor: pointer;
  -webkit-appearance: button;
  font-size: 100%;
  margin: 0;
  max-width: 100%;
  vertical-align: baseline;
  line-height: 1.5;
  display: inline-block;
  align-items: flex-start;
}

.screen-reader-text {
  clip: rect(1px, 1px, 1px, 1px);
  position: absolute !important;
  height: 1px;
  width: 1px;
  overflow: hidden;
}

.sidebar-toggle:before {
  content: "\f419";
  height: 24px;
  width: 16px;
  color: inherit;
  font-size: 16px;
  line-height: 24px;
  speak: none;
  text-decoration: inherit;
  vertical-align: top;
  font-style: normal;
  font-weight: normal
    display: inline-block;
  font-family: Genericons;
  -webkit-font-smoothing: antialiased;
}

.site-branding {
  width: 95%;
  display: inline-flex;
  margin-top: 0;
  margin-bottom: 0;
  float: left;
  max-width: 100%;
}

#sitelogo {
  display: inline-flex;
  vertical-align: middle;
  text-align: center;
}

.site-title {
  border: 3px solid grey;
  font-family: Impact, Charcoal, sans-serif;
  font-weight: normal;
  margin: 0 auto;
  text-align: center;
  line-height: normal;
  min-width: 150px;
  padding: 3px 8px;
  font-size: 6vh;
  height: 100%;
  color: white;
  float: left;
  max-width: 100%;
}

h1 {
  clear: both;
}

.site-description {
  background: #fff500;
  color: black;
  display: block;
  font-family: Impact, Charcoal, sans-serif;
  margin: auto auto;
  text-align: center;
  height: 100%;
  border-left: none;
  border-top: 3px solid grey;
  border-bottom: 3px solid grey;
  border-right: 3px solid grey;
  display: block;
  padding: 0 16px;
  font-size: 3vh;
  line-height: 2.3;
  float: left;
  clear: none;
}

<header id="masthead" class="site-header" role="banner">
  <button class="sidebar-toggle" aria-expanded="false" ><span class="screen-reader-text"><?php _e( 'Toggle Sidebar', 'boardwalk' ); ?></span></button>
  <div class="site-branding">
    <div id="sitelogo" class="clear">
      <h1 class="site-title"><a href="http://test.com" rel="home">My Website</a></h1>
      <h2 class="site-description">Your Go To Stop</h2>
    </div>
  </div>
</header>