This does not look like downy mildew to me. I am in agreement with the above answer regarding sunburn/scald. If leaves were covered up by anything or shaded for a while and then abruptly exposed to harsh sunlight, or there were several dark or rainy days followed by strong sun, there can be some sunburn on the leaves. A similar symptom can be seen from mild frost/cold damage, but due to your location and broccoli's status as a fairly hardy plant, I doubt that is the case here.

My biggest issue with broccoli anymore is clubroot, which manifests as swollen, knotty growths on the roots. Above ground, plants wilt severely on hot or dry days, and their growth slows or ceases. They may also head prematurely.

If the only problem is sunscald/burn, the plants will likely recover and go on to produce well. If there is another problem, they may continue to decline. Be vigilant for additional symptoms, as this can help determine any additional issues.