In HTML/CSS you're referring to a relational display model
Where elements are positioned based on their relationship to the elements that come before/after them in the tree.
The alternative is an absolute positioning model where the structure doesn't matter and every element's position is explicitly placed in relation to the top left corner.
Relational is generally preferred because it allows you to make changes to parts of the whole without requiring a full re-calculation of all the offsets within the page.
For example, if you change the offset for a panel that is 5 generations/nodes removed from the main panel; in a relational mode, everything re-flows to compensate automatically; in an absolute model, the offset for all 5 generations/nodes of parent panels need to be re-calculated in cascading order because no child can calculate it's offset in relation to its parent until the parent is updated first.
The tricky part about HTML/CSS layout models is, you can mix them.
For instance, a 5th child panel can be placed using relative positioning (display:relative) but the child elements of that node can be placed using absolute positioning (display:absolute) within that element. Ie Instead of calculating the top-left offset from the window corner, it's taken in relation to the top-left of the box it resides in (5 generations deep).
The power of the relative positioning model is that no child has to be aware of its parent's positioning. Child elements can be blissfully unaware. The difficulty comes in when there are issues/inconsistencies (ie as has happened a lot with browser inconsistencies in the past). Then you'll have to crawl back up the tree one element at a time until you can determine the one causing the trouble.
Fortunately, with tools like FireBug and Google Chrome Developer Tools, it's really easy to crawl through the tree and visualize the layout boxes, padding, margins, etc interact with each other.
Unfortunately, once you adopt a relational model it becomes really difficult to design a typical drag-drop GUI builder tool so most relational GUI development happens in code only. Maybe that will change in the near/distant future.
Note: For completeness sake, I feel like I should also mention the fixed positioning model. It's basically the same as absolute positioning but it is not affected by scrolling. That's how those annoying toolbars that stick to the bottom/side of the page are created on most websites.