progress: known and unknown progress

Useful to know

The <progress> element represents task progress. With value="3" and max="10", it describes three completed units out of ten. Omitting value makes it indeterminate: the task is running, but no completed proportion is specified. [1]

For uploads, show a percentage only when the application receives actual progress data. An animation reaching 90% on a timer is not a measurement of transferred bytes. If the total size or duration is unknown, the indeterminate state communicates that more accurately.