1/HTML ads dramatically reduce the time, cost and complexity of running A/B tests because of how they are served. To explain this, think of a campaign with 5 headlines and 5 product images, across 5 languages. That makes 125 variations of the ad you need produced and proofed. Ouch! Whilst Flash is served as a compiled file (you collect all the bits together when you make it), so doing this test in Flash would mean creating 125 individual files. HTML on the other hand can be served in bits. That means running this example test would involve creating only 1 ad creative with 2 variables.
2/HTML ads make dynamic content much easier! Your ads can change in real time. For example, e-commerce stores, events or gambling companies can up prices, tickets or betting odds as they occur, keeping the advertising totally up to date.
3/HTML can be altered by accessibility programs on computers. This also means they can be understood by devices like screen readers (which can read the text to the visually impaired).
4/CSS3 can be responsive, sneaking another point here over flash ads. – meaning ads can fit any screen size with a single file. The same ad would work on home connected theatre screen and likewise different sized tablets. Furthermore, responsive HTML ads can make best use of space across any device. At last! Ads that will work on mobile phones and tablets. Move over flash.
5/Finally, HTML ads have a much smaller file size so they load much faster than flash.