Mark Miller has a nice writeup about Smalltalk and Seaside, a good collection of resources. One minor note, he mentions while referring to Seasides lack of HTML templates...
"They must be set up as strings within your Smalltalk code, as part of the whole Seaside application".
This isn't true at all, Seaside includes a complete domain specific language in native Smalltalk for generating HTML, there is no string handling involved. You build HTML with tags and attributes, just as you do in HTML.
Smalltalk is fully capable of expressing the same abstract syntax tree of tags and attributes that HTML expresses, so there's no need for HTML templates. Smalltalk's syntax is superior to HTML's syntax. It's not that Seaside doesn't have templates, it's that it doesn't need them.
html div class: #Header; style: 'visibility:hidden;'; with: [html span: 'Hello World']
the Smalltalk above is just as expressive as the HTML below, and far more manageable.
<div class="Header" style="visibility:hidden"><span>Hello World</span></div>
When using the Canvas API, you can write your pages, with the full power of Smalltalk's tools at your disposal. More importantly, you can do something you can't do with templates, factor your code into reusable methods, and call them. It's vastly easier to do complex layouts and more importantly, maintain them, in Seaside, than in a giant unfactored HTML template.