When copying the embed code from YouTube, a fixed width and height is given for the iframe. This will probably look ugly on web pages with responsive designs. As iframes are not images, there's no way for the browser to know what height to set the frame to. Fortunately, all YouTube embeds are of 16:9 ratio, and we can work around this with a little bit of CSS.
HTML snippet
<div>
<div style="position:relative;padding-top:56.25%">
<iframe src="<youtube-embed-url-here>" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe>
</div>
</div>How this works
Notice that we are not using the width and height properties, instead we are using CSS style. Two important things makes it work, the first padding-top:56.25 creates a 16:9 aspect ratio div box for the iframe to fill up. Next, we set the iframe's style to position:absolute;top:0;left:0;width:100%;height:100%;. This positions the iframe at the top-left corner of the div block and sets it occupy the entire block which is of the correct aspect ratio. Now we have an auto-resizing embedded block that's filled with the desired YouTube video!