This method is very simple for anyone comfortable editing basic CSS and HTML. However, it does require some work for each individual YouTube embed. This is less than ideal, especially for websites with multiple authors and editors who likely all try to embed YouTube videos in pages and posts.
Method 2: With Gutenberg Blocks in responsive themes
Some WordPress themes, such as Inspiro and responsive, have features that automate responsiveness for video embeds. So if you’re open to changing your