Bamboo could be right about the spider mites, but another possibility is the soil pH might be too high, which could make manganese less available and cause deficiency symptoms, which sometimes look similar to your basil plant's leaves. If the plant continues to grow at the same rate, I doubt soil pH is the issue, though. Adding phosphorus is said to increase the availability of manganese. I'm not sure how true that is. Acidifying the soil increases the availability of manganese (but it can be difficult to do in containers. Adding extra manganese probably won't help unless it's not a pH issue and the soil is manganese deficient.
Also, thrips can cause stipling on leaves, too. Scale insects can sometimes make similar damage.