People use web apps with wildly different designs all the time because there are no UI standards and there is no basic toolkit. It's a bug, not a feature.
I'm not sure that's right... would the web have caught on and be as well-loved if everything used "system" colors and designs? Maybe you're envisioning something different, but I'm not sure where the middle ground is between the web as it exists today and a classic Windows 3.1 / Mac looking app - what would a "basic toolkit" look like beyond the HTML elements and form controls that we have?
The thing is that, to the extent that it is a bug, it's a very harmless bug. The freedom to experiment and innovate with web apps has probably outweighed any harm from lack of consistency.
I'm not sure that's right... would the web have caught on and be as well-loved if everything used "system" colors and designs? Maybe you're envisioning something different, but I'm not sure where the middle ground is between the web as it exists today and a classic Windows 3.1 / Mac looking app - what would a "basic toolkit" look like beyond the HTML elements and form controls that we have?