Separation of Presentation and Content
Summary: One reason to separate style from content is to reuse HTML or CSS. Ultimately, we would like a solution where we can reuse both.
There is an economic reason to separate presentation from content. Publishers have thousands of pages of HTML on their site, yet they want to enhance the style of their pages over time. It would cost a lot of money to change every single page to match their new style. So they invest a little more time writing each page so that the HTML markup does not refer to styles but to the semantics of the content (referred to as semantic HTML). Then they hire a designer to write CSS to make their existing content look new. The HTML is permanent and reusable, and the CSS is temporary and not-reusable. The separation is only one way: the HTML doesn't know the CSS, but the CSS does know the HTML.
Examples: CSS Zen Garden, newspaper websites, blogs
Characteristics: Semantic markup, CSS tailored to classes/structure of HTML
Yet another economic reason is a relatively newer phenomenon. It has become very easy to create a new web site/application. Writing (or generating) lots of HTML is cheap, and it changes often during iterative development. What is relatively expensive is to design each of those pages each time the pages change. CSS is not good at adapting to page structure changes. So people have built CSS frameworks where the CSS is (relatively) permanent and the HTML is temporary. In these cases, the HTML knows the CSS, but the CSS doesn't know the HTML. The separation is again one way---this time the other way.
Characteristics: HTML tailored to classes/structure of CSS, Reusable CSS
Reusable Content and Styles
What if a newspaper site, with millions of existing HTML pages, could cheaply take advantage of the reusable styles of frameworks like Bootstrap? That is the Holy Grail of separation of concerns. What would be required to do that?
What we really want is a two-way separation. We want HTML written in total isolation and CSS written in total isolation. We want permanent HTML and permanent CSS. How can the style and content, each developed separately, finally be brought together? The answer is simple: a third document to relate the two.
We have already seen that CSS is not good at abstraction. CSS cannot name a style to use it later. However, LESS does have powerful forms of abstraction. LESS has the ability to define reusable styles and apply them to HTML that did not have those styles in mind. If you put the definition of reusable styles in one document and the application of those styles in another document, you achieve true separation. And it is already happening a little bit. You can do it in your own code.
It is a bit like a software library. We put the reusable bits in the library, and their specific use in the app.
Characteristics: Semantic markup, Reuseable Styles, Tie-in document to relate Style to Content
CSS preprocessors, which began as convenience tools, are actually powerful enough to solve fundamental problems with HTML and CSS. While it is still early, LESS and other CSS preprocessors, if harnessed correctly, could dramatically transform how we build and design web sites. Typography, grids and layout, and other design concerns can be used as plugable libraries. And other languages that are specifically designed to do that may emerge. What would a systematic, analytical approach to such an approach look like?