This isn't necessarily specific to DigitalOcean, as I am experiencing the same thing with S3.

First, here is the request that I am sending successfully from Node.js AWS SDK:

s3.putBucketCors({
  Bucket: EXAMPLE_BUCKET,
  CORSConfiguration: {
    CORSRules: [
      { 
        AllowedHeaders: [`*`],
        AllowedMethods: [`GET`],
        AllowedOrigins: [`https://example.com`]
      }
    ]
  }
}, (err, data) => {
  if (err) throw err
  console.log(data)
})

I have verified that this indeed works as a call to getBucketCors returns expected configuration.

From the above policy, I would expect to not be able to access objects in EXAMPLE_BUCKET from localhost, or anywhere that isn't example.com, however, that isn't the case. I am able to GET objects in this bucket from localhost, as well as http://dev.example.com.

What am I not understanding?