Blocking at the request level is where it's at: you don't download the content, you don't execute the scripts, you don't interact with the DOM. You block requests to third party servers or resources based on lists or other policies. This saves you load time, execution time, and it keeps you from being tracked by targeted ad systems.
If people start bypassing the DOM with canvas we will have a bigger problem than cosmetic filters failing: we will no longer be served the same kind of document. Or we will, but the content will be in an opaque region of it. If that happens the web is dead. People tried this before with flash and that smoldered for a while in the 90s but didn't last, so I'm not feeling all doom-and-gloom about it.
Aside: Standardized document formats are important. If rendering to canvas takes on as the defacto standard, it would cripple the web technologically.
If people start bypassing the DOM with canvas we will have a bigger problem than cosmetic filters failing: we will no longer be served the same kind of document. Or we will, but the content will be in an opaque region of it. If that happens the web is dead. People tried this before with flash and that smoldered for a while in the 90s but didn't last, so I'm not feeling all doom-and-gloom about it.
Aside: Standardized document formats are important. If rendering to canvas takes on as the defacto standard, it would cripple the web technologically.