afrontend/manifest.json

## manifest.json
{
    "manifest_version": 2,
    "name": "Reading Web Data Example",
    "description": "This extension shows a current temperature of the seoul",
    "version": "1.0",
    "browser_action": {
        "default_popup": "popup.html"
    },
    "permissions": [
        "https://www.accuweather.com/"
    ]
}

## popup.html
<!DOCTYPE html>
<html>
  <head>
    <title>서울 기온</title>
    <meta charset="utf-8" />
    <style type="text/css">
    .box {
      width: 120px;
      height: 14px;
      line-height: 14px;
      font-size: 14px;
      color: brown;
    }

    .box > span {
      color: red
    }
    </style>
    <script src="popup.js"></script>
  </head>
  <body>
    <div class="box">서울의 온도 <span id="temperature">0 도</span></div>
  </body>
</html>

## popup.js
document.addEventListener('DOMContentLoaded', function () {
  httpRequest = new XMLHttpRequest();
  httpRequest.onreadystatechange = getContents;
  httpRequest.open('GET', 'https://www.accuweather.com/ko/kr/seoul/226081/weather-forecast/226081');
  httpRequest.send();

  function getElement(html) {
    var div = document.createElement('div');
    div.innerHTML = html;
    return div;
  }

  function parseHTML(html) {
    var el = getElement(html);
    var temp = el.querySelector('#current-city-tab > a > span.local-temp').innerHTML;
    document.querySelector('#temperature').innerHTML = temp;
  }

  function getContents() {
    if (httpRequest.readyState === 4) {
      if (httpRequest.status === 200) {
        parseHTML(httpRequest.responseText);
      } else {
        document.querySelector('.box').innerHTML = '온도 읽기 실패';
      }
    }
  }
});
	{
	"manifest_version": 2,
	"name": "Reading Web Data Example",
	"description": "This extension shows a current temperature of the seoul",
	"version": "1.0",
	"browser_action": {
	"default_popup": "popup.html"
	},
	"permissions": [
	"https://www.accuweather.com/"
	]
	}