A recent study performed by Pacific University College of Optometry looked into precisely this question. The study collected spectacles ordered from various online vendors, and were tested for optical tolerance and impact resistance requirements for eyewear dispensed in the United States. In their testing they purchased 200 pairs of spectacles from the 10 most popular web-based eyewear dealers.
The spectacles ranged in different lens and frame materials, lens styles, and different prescriptions, representing common refractive conditions. After all the spectacles were received they were tested on correct prescription such as sphere power (near-sightedness/far-sightedness), cylinder power and axis (astigmatism), add power (bifocal), prism imbalance, and impact testing.
Of the 200 pairs ordered, the College only received and evaluated 154 pairs, totaling 308 lenses. Several of the spectacles were entirely incorrect, such as single vision received instead of a multifocal and lens treatments added or subtracted (scratch coating, non-glare, etc.). One lens failed tolerance standards for at least 1 optical parameter 28.6% of the time and 22.7% of glasses failed the impact resistance testing (safety lenses). Overall, 44.8% of spectacles failed to meet 1 parameter of optical or impact testing.
When it comes to your vision and eye safety it is important to trust the people you order from. Drs. Hawks, Besler, & Rogers provide premium products and testing with every spectacle purchase.