[trnglina]

And for anyone who isn't familiar with (what I would consider) the most important difference in this use-case: borders affect the positioning of other elements, whereas outlines do not. You don't want to be using a temporary border, since it can significantly affect the layout of your site when you remove it.

[mdtusz]

* { box-sizing: border-box; }

I have never wanted anything other than this behaviour, so I pretty much always include it when starting on a fresh project.

[adamwathan]

Borders still affect the positioning of things even with box-sizing set to border-box. For example, here "Block 1" is still two pixels taller than "Block 2":

<div style="padding: 8px; border: 1px solid black"> Block 1 </div>

<div style="padding: 8px;"> Block 2 </div>

https://jsfiddle.net/t4rkvfzx/

[GrantSolar]

Does that really matter though? A difference of 2px for something temporary doesn't seem like much concern. While I understand the implication is that these can accumulate and cause larger layout issues, the purpose of this practice is to highlight those areas to remove that accumulation anyway

[ComputerGuru]

The bigger issue is that you don't know if a UI inconsistency is a bug or a side-effect.

[_bxg1]

For this case of debugging, though, borders might actually be preferable because outlines would overlap, potentially obscuring the DOM depth

[Jaruzel]

When I'm coding DIV layouts by hand, I temporarily assign each DIV with a random background-color. Once I have the layout and responsiveness required, I strip the background-color setting on each DIV as I fill them up with content. Works for me.

[whalesalad]

Yep, that is exactly why I do this! Great explanation.