Question: Because of certain reasons, I'm sometimes tied to an extremely slow internet connection. What I realized with this, is that certain sites (for example, any subdomain of blog.hu) works in a way that needs a certain part of the site be loaded before comments are loaded too. I assume it's because of an AJAX operation, as the comments themselves are not visible within the HTML file.

In contrast, forum engines, such as vBulletin are often free from AJAX, meaning that when a thread's HTML is loaded, every comment in it are "hardcoded", meaning that even if the "fancy parts" (CSS, images and so on) are not loaded, I can already read the comments.

My needs are very special though, so I'm interested - is there any general UX advantage of any of these?

Answer: 
@JoshDoebbert has a good point on the text legibility. [This other question](https://ux.stackexchange.com/questions/34120/relationship-between-font-size-and-width-of-container) has some interesting information on the topic.

I just wanted to add that percentages are good to let the content adapt to smaller screen size. As you said there is a minimum size and in the same way there is maximum size that lets the content be understood correctly. Whether it is text or images **the screen size in both height and width should be considered to display the content correctly**, and set relevant breakpoints for both parameters.

[![enter image description here](https://i.stack.imgur.com/oTPat.jpg)](https://i.stack.imgur.com/oTPat.jpg)
* **Content Display**

Think about this: you make a site with content prepared for full width, no max-width. This will work relatively OK for common sizes, but full screen images for big screens will add a massive load OR they will get pixelated. Just choose your poison. And this is easily fixed by... max-width

* **Legibility**

There are many studies about line length. [NN/G says between 45 to 75 characters](https://www.smashingmagazine.com/2014/09/balancing-line-length-font-size-responsive-web-design/), [Baymard proposes 50 to 75 characters](http://baymard.com/blog/line-length-readability), [Viget says around 100 characters](https://www.viget.com/articles/the-line-length-misconception) and so on. For example, this post has around 100 characters width, and it takes 1/3 of my screen. The whole idea of having a 300 characters line makes me dizzy!

* **Control**

Your screen is a container which holds different elements. When you build a layout, you will place those elements according to different design theories, your project needs, your users, the needed elements to be displayed and so on. Thus, there's a high degree of intentionality. If you leave this to pure luck, all these elements will be randomly scattered and resized with unpredictable results. For example: An *"above the fold"* button may need 2 scrolls after an image is resized to adapt to full width (and proportionally increase height). As you may imagine, this isn't a very good option for anyone

* **Focus**

This is related to the item above. For example, let's say you want your user see a [smiling person right in front of her eyes](http://www.psychologicalscience.org/observer/the-psychological-study-of-smiling#.WEC5RH2S5aQ), and a CTA below that to increase clicks. Works great on mobile, laptops and desktops up to 1200px width. Then you decide to allow full width. Now the smile could be at the middle/bottom of the image and your CTA completely lost. Any simple study will tell you this is extremely wrong, and it's easily measurable

---

**Finally, one thing:** Full width with some amount of control is OK for carefully crafted sites with no dynamically generated content. However, you need to fine tune this in many forms. 

You gave the Google example: well, the **header takes the whole width**, but the content takes only `600px width` (the results themselves) with a maximum width of `1250px` if right sidebar is displayed. **See image below:**

[![enter image description here](https://i.stack.imgur.com/rLKaY.jpg)](https://i.stack.imgur.com/rLKaY.jpg)