JSFiddle - React, Tailwind, and code Playground

HTML

<form class="h-adr">
        <span class="p-country-name" style="display:none;">Japan</span>

        <div class="contact">
          <div class="contact-item">
            <div class="contact-heading">郵便番号</div>
            <div class="contact-detail contact-detail-postcode">
              <span class="zipcode-mark">〒</span>
              <input type="text" class="p-postal-code" size="3" maxlength="3">
              <span> - </span>
              <input type="text" class="p-postal-code" size="4" maxlength="4">
            </div>
          </div>
          <div class="contact-item">
            <div class="contact-heading">都道府県</div>
            <div class="contact-detail contact-detail-address">
              <select class="p-region-id">
                <option value="">-- 選択してください --</option>
                <option value="01">北海道</option>
                <option value="02">青森県</option>
                <option value="03">岩手県</option>
                <option value="04">宮城県</option>
                <option value="05">秋田県</option>
                <option value="06">山形県</option>
                <option value="07">福島県</option>
                <option value="08">茨城県</option>
                <option value="09">栃木県</option>
                <option value="10">群馬県</option>
                <option value="11">埼玉県</option>
                <option value="12">千葉県</option>
                <option value="13">東京都</option>
                <option value="14">神奈川県</option>
                <option value="15">新潟県</option>
                <option value="16">富山県</option>
                <option value="17">石川県</option>
                <option value="18">福井県</option>
                <option value="19">山梨県</option>
                <option value="20">長野県</option>
                <option value="21">岐阜県</option>
                <option value="22">静岡県</option>
                <option value="23">愛知県</option>
                <option value="24">三重県</option>
                <option...

CSS

@charset "UTF-8";

p {
  line-height: 2;
}

.header {
  width: 100%;
  background-color: #fff;
}

.heading {
  font-size: 2rem;
  text-align: center;
  padding: 20px 0;
}
@media screen and (max-width:767px) {
  .heading {
    font-size: 1.5rem;
  }
}

.container {
  padding: 20px 0;
  max-width: 960px;
  margin: auto;
}

.heading-title {
  font-size: 1.875rem;
  overflow: hidden;
  text-align: center;
  margin-bottom: 20px;
}

.heading-title-inner {
  position: relative;
  display: inline-block;
  margin: 0 2.5em;
  padding: 0 1em;
  text-align: left;
}

.heading-title-inner::before,
.heading-title-inner::after {
  position: absolute;
  top: 50%;
  content: '';
  width: 400%;
  height: 1px;
  background-color: #ccc;
}
.heading-title-inner::before {
  right: 100%;
}

.heading-title-inner::after {
  left: 100%;
}

/* 郵便番号エリアのCSS */
.contact {
  margin: 20px auto 50px;
  border: 1px solid #dedede;
}

.contact-item {
  display: flex;
}
.contact-item:not(:last-child) {
  border-bottom: 1px solid #dedede;
}

.contact-heading {
  display: flex;
  justify-content: center;
  align-items: center;
  min-width: 8em;
  width: 20%;
  padding: 10px;
  background-color: #f3f3f3;
}

.contact-detail {
  width: 80%;
  padding: 10px;
}

.zipcode-mark {
  padding-right: 10px;
}

.contact-item input[type="text"] {
  box-shadow: inset 1px 1px 3px rgba(0,0,0,.1);
  border: 1px solid #ccc;
  padding: 5px;
  background-color: #fefefe;
}

.contact-detail-postcode input {
  width: 8em;
}
@media screen and (max-width:767px) {
  .contact-detail-postcode input {
    width: auto;
  }
}

.contact-detail-address input {
  width: calc(100% - 10px);
}