The ProgressEvent has attributes `loaded` and `total` indicating the progress, and their type is `unsigned long long` now. With this feature, the type for these two attributes is changed to `double` instead, which gives the developer more control over the value. For example, the developers can now create a ProgressEvent with the `total` of 1 and the `loaded` increasing from 0 to 1 gradually. This is aligned with the default behavior of the <progress> HTML element if the max attribute is omitted.
https://github.com/webmachinelearning/writing-assistance-apis/issues/15
Explainers: https://github.com/whatwg/xhr/pull/394