Marissa Mayer, Chief Executive Officer of Yahoo, Co-Chair of the World Economic Forum Annual Meeting 2014, attends a session at the annual meeting of the World Economic Forum (WEF) in Davos January 24, 2014. REUTERS/Denis Balibouse

At a panel put on by Salesforce.com here at the World Economic Forum in Davos, Yahoo CEO Marissa Mayer just said she and her teams have come up with a new design rule to make sure every app they build is "fast, responsive, and beautiful."

It's called the "two tap rule."

The test for the rule is simple, says Mayer: "Once you’re in the app, is it two taps to do anything you want to do?"

If yes — the app is a go. If no, it's back to the drawing board.

When I was doing reporting for my new book, "Marissa Mayer and the Fight to Save Yahoo!," I learned that, among her colleagues at Yahoo and her ex-colleagues at Google, Mayer is famous for these kinds of rules.

She developed them during her time running Google's user-interface product reviews, when every product the company launched had to be approved by her first.

Once, during her Google tenure, an associate product manager brought a prototype for her review, and she said to him, “This page is too busy. What you need to do is look at every font on the page, every font size. And every time you see a new color or a new font size, you add up a point. I want this page below five points.”

The comment about “five points” ended up in the meeting notes, and then it became a rule. Google would no longer launch Web pages with more than five points.

Another rule Mayer created during her Google years, and has continued onto Yahoo, is what could be called the 98% rule.

Mayer mandates that her people design a product for the way it will be used 98% of the time.

IFor Mayer, the best example of a product that followed this rule was the Xerox copy machine. It could do all kinds of fancy things: staple, collate, copy, and fax. But if you walked up to one and pressed the giant green button, the right thing just happened.

Mayer believes on every good product there should be a big button like that for the 98 percent use case, where if the user clicks it or taps it, they get a delightful, fluid, simple experience.

If you're fascintated by Marissa Mayer's design rules and how she rose to the top at Google, then you are the exact kind of person I hope would read my book. You can buy it on Amazon now.