If the network speed is too fast, we cannot check whether it works or not. So, I recommend you to make the speed slower. Below is the ways according to browsers.
In Chrome DevTool: Please go to [Network > No throttling]. And then, choose the speed that you want.
In Firefox DevTool: Please go to [Network > No Throttling]. And then, choose the speed that you want.
I recommend you to copy the codes to your own files and execute them rather than execute them on the online IDE. That's because in online IDEs, other parts should be loaded too.
The contents can be cached. So, I recommend you to do hard refresh.
If you set the background color in <img> tag, the color will be shown until the image is loaded.
In the below example, I used a 'template' tag.