It depends on the context.
In the past, browsers image resize quality was very poor. Modern browsers can resize most images quite well. However, resizing a very large image (i.e. 2000x1000) to a very small size (i.e. 200x100) may result in poor quality. As an aside, it is usually discouraged to upscale images as quality will suffer.
From a network (downloading/bandwidth) perspective, it is often discouraged. If an image is 800x600 and is sized to 640x480, the user will still need to download the 800x600 version in order to see the HTML sized 640x480 version. This is extra load on your server and increases the download time for the user.
Scaling images in HTML/CSS is most commonly used properly in the following contexts:
Responsive design. Based on the min-width you are using, I am guessing that this is the circumstance. If the site scales, images almost always have to scale as well. If an image must scale hundreds of pixels, you may consider swapping the image for a high/lower resolution image as needed.
Mobile-optimized websites. A 640x480 image HTML/CSS scaled to 320x200 will appear much sharper on a high-resolution (i.e. Retina) mobile device. Mobile devices have a very high PPI (pixels per inch) and can take advantage of the extra data. In fact, a 640x480 image rendered at 640x480 can appear blurry (in comparison to text or the OS interface) if the website is not scaled down to fill the device screen.