When using S3 to server web site content (both in conjunction with CloudFront and not) you will may want to set Cache Control headers for browsers or CloudFront to use.
$ aws s3api put-object --bucket mybucket --key index.html --cache-control max-age=3600 --body www-files/index.html
{
"ETag": "\"63525d1b04aeb5d301de2acc498480b3\"",
"ServerSideEncryption": "AES256"
}