We need better Encapsulation
CSS took an important step towards separating the content of an HTML document from its presentation. But significant parts of web page's design cannot be described by CSS, and are instead defined by design HTML interleaved with content HTML.
This Design HTML is stylistic scaffolding. It provides anchor points for CSS and controls block-level layout beyond CSS's capabilities. But unlike CSS, it cannot be separated from content HTML. This makes web documents harder to create, maintain, and reuse.
Template languages address symptoms of this problem, but not the root. We need a platform-level language to support encapsulation and reuse of HTML structure just like we have one for HTML style.
Enabling Content Reuse
Cascading Tree Sheets (CTS) is a light-weight language for describing the presentational parts of page design that CSS cannot represent. CTS looks just like CSS, but it describes the way HTML fragments should bind with, and replace, other HTML fragments.
Tree Sheets are attached externally, like CSS. This means your web templates are actually fully-functioning mockups. A tree sheet transforms the mockup into a page with custom content.