<img id="image" src="https://dl.dropbox.com/s/b2g16cs6hlu3wnc/img01.jpg?dl=0">
*{
  text-align:center;
}

img{
  max-width:50%;
}
window.addEventListener('load',function(){
  const elem = document.getElementById('image');
  elem.insertAdjacentHTML('afterend',`<p>画像の横幅：${elem.width}`);
});

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External JavaScript

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