What if Jakob Nielsen had a blog?:

It's good when a tab click takes effect immediately, changing the panel area without a full page reload. But not all within-the-page updates are good.
On the Yahoo Finance page, stock quotes are updated in real time and each change is announced by a color flashing behind the affected number. The constant flashing all over the screen gets tiring fast, especially for numbers like volume, which change incessantly.

Just because you can do something doesn't mean that you should. Yes, it is a guideline to draw users' attention to updates within a page, but only when users need to be alerted. In this case, all users need to know is that all numbers are current and will be continually updated.

A trading system for day traders would be a different matter. In that scenario, users need to continuously monitor market trends for multiple securities, and colors flickering in their peripheral vision could alert them to short-lived trading opportunities.

On a financial portal homepage, however, users have to scan a broad set of headlines and stories. If they want to trade, they go to their broker's site, so they couldn't take advantage of anything that required trigger-fast actions anyway. Few people would sit and stare at this page to keep up with second-by-second changes; the relentless blinking is overkill and distracts users from the content they came for.

http://www.useit.com/alertbox/tabs.html

So what good are usability guidelines if — as this example seems to show — they are always in conflict?
Actually, the guidelines don't contradict each other when it comes to the Census homepage. (Sometimes we do have conflicting guidelines, as in the matter of intranet staff directories. That's why we call them guidelines: because they aren't always firm and sometimes require interpretation or, in rare cases, deviation.)

In this case, though, it's possible to feature the high-priority tasks, show the main number directly on the homepage, and have a tool for the rest without using marketing-style terms, heavy formatting, and otherwise making the important area look like an ad or a promotion. That is, the designer could have followed all 6 guidelines simultaneously.

In many studies of corporate and e-commerce sites, we observe users ignoring the very page sections that have the most business value because the site is selling them too hard. Learn from the Census Bureau's mistake: try a softer approach with more straightforward, user-centered information. Let go of the fancy formatting and the fancy words.

http://www.useit.com/alertbox/fancy-formatting.html