I think you could also use a nine-patch. I know they are used in android dev, but I found this guy on stack overflow that says they can be used on the web too. Check it out here
https://stackoverflow.com/questions/3659457/nine-patch-images-for-web-development
and here’s a website made by Romannurik to easily create nine-patches
https://romannurik.github.io/AndroidAssetStudio/nine-patches.html#&sourceDensity=320&name=example