The Interactive Advertising Bureau primarily exists to help grow the pie of digital advertising dollars. So, as it kicks off its annual leadership meeting this week, it's focusing on where that growth is expected to occur: mobile.

Today, the IAB -- along with a group of publishers including AOL, Forbes, Conde Nast, Time Inc. the Wall Street Journal, The New York Times and more -- released a letter urging advertisers to design in HTML5, a format it said would ensure ad creative renders appropriately across all screens and improves the mobile ad experience.

"You should insist to your ad agencies that your advertising creative be developed in a mobile-compatible format," the letter reads. "And the one open, industry-standard, universal format for building mobile-ready creative is HTML5."

Anna Bager, VP and general manager at the IAB's Mobile Marketing Center of Excellence, said designing better, more effective cross-sceen ads "will also simplify the process and improve the value of the advertising, thereby increasing the amount if ad dollars."

The full letter, also signed by Conversant, Google, IDG, Millennial Media, New York Post, Slate, Time Inc., The Weather Company and Xaxis, is below:

Dear Advertisers,

We'll just say it: It's time to improve your mobile advertising.

In a medium that's all about movement, your ads are inert. With consumers accustomed to content tailored to their time, place and interests, most mobile ads are peculiarly one-dimensional.

To guarantee the ads you pay for actually appear and look great on all screens, you should insist to your ad agencies that your advertising creative be developed in a mobile-compatible format. And the one open, industry-standard, universal format for building mobile-ready creative is HTML5.

Your opportunity has never been greater. Nearly half of the US population has a mobile phone with internet access*, and one out of five pageviews on the web happen on a mobile device - a number that is growing every month.**

HTML5 will enable you, your agencies and publishing partners to make your creative ideas captivating on every screen, elevate your brand image, and lower your creative costs. We know this may mean a change in how you develop your ad creative. But there are many tools to augment work with HTML5, such as responsive design, and a multitude of suppliers available to assist.

If you're not familiar with HTML5, IAB offers guidance on optimizing HTML5 ad code for better performance across platforms: HTML5 for Digital Advertising 1.0: Guidance for Ad Designers & Creative Technologists. In addition, IAB, in partnership with technology leaders in the space, will be producing a series of webinars focused on HTML5 and other mobile advertising solutions. To learn more, register at iab.net/MakeMobileWork.

Then ask your agencies and developers for mobile-friendly HTML5 so you can reach your audience across all screens.

Sincerely,