Fairly allocating distributed computing resources among workflow executions is critical to multi-user platforms. However, this problem remains mostly studied in clairvoyant and offline conditions, where task durations on resources are known, or the workload and available resources do not vary along time. We consider a non-clairvoyant, online fairness problem where the platform workload, task costs and resource characteristics are unknown and not stationary. We propose a fairness control loop which assigns task priorities based on the fraction of pending work in the workflows. Workflow characteristics and performance on the target resources are estimated progressively, as information becomes available during the execution. Our method is implemented and evaluated on 4 different applications executed in production conditions on the European Grid Infrastructure. Results show that our technique reduces slowdown variability by 3 to 7 compared to first-come-first-served.