If it's only static HTML and CSS, it's just a bucket o' files. You can store things like that on S3 for... I don't even know how little. A tiny amount. You will not find a cheaper hosting option than this.
I made this site on S3, using Svelte, which is intentionally the humblest, smallest step up from plain JavaScript HTML & CSS, at the link below.
http://pitch-deck-svelte.s3-website-us-east-1.amazonaws.com/
Now, the one drawback here is that this doesn't have HTTPS, and you'd need CloudFront for that. This link covers how to do that (note: you should buy your domain on AWS, using Route 53, when following these instructions to make your life easier).
https://medium.com/@sbuckpesch/setup-aws-s3-static-website-h...