CSS Margin, Gap, and Flexbox

Written by Human, Not by AI

I like the CSS box model.

box model representation

It makes sense. There’s only one issue: that’s not it.

The CSS box model actually has the borders on the outside of the box. So when you add a border, it will overflow the element!

diagram showing how borders extend outside of the parent

As developers, we get the box-sizing: border-box rule to solve this issue.

We rarely come across the case where the borders of an element are so large that they cause our layouts to break, so box-sizing: border-box (despite being a more sensible default) is often forgotten.

However, there is one other (much more common) property that is so ubiquitous that its odd side effects are unquestioned. The CSS margin property’s tendency to push sibling elements around is a squabble that we seem to largely put up with as developers.

Margins are most often used to add arbitrary amounts of space between elements to make them feel right together. There is often no rhyme or reason to the margins we add to elements.

The idea that there is a CSS property that we arbitrarily tinker with until the page looks correct seems fundamentally at odds with the way we architect web applications. Inevitably, margins worm their way into our classes and components. This makes them less reusable. An element’s margin should be a property of the particular composition of elements, not the element itself.

What if we didn’t use margins

Perhaps the most simple solution to margin woes is to never use them. One must resist the urge to replace their margins with padding, as padding has a much different purpose and _makes sense to be _a property of the element. A property much more fit for purpose (and promising!) is gap.

gap is a property set on a grid or flex parent. It spaces elements out evenly with a gap between them. I think CSS gap might be my favourite property. I love it.

Unlike margins, gap is a property of a group of elements. This makes it very well suited to margin’s job—and I think it should almost always replace margins in situations where a uniform gap is needed between elements. However, as much as I wish I could set gap on elements using the block layout algorithm, the added noise of display: flex | grid can muddy up CSS classes a bit—but it’s a price I’m willing to pay.

gap is great until you come up with the following bright idea:

<article style="display: flex;flex-direction: column;gap: 1rem;">
  <h1>Hello my friends</h1>
  <p>This is a call to all my friends</p>
  <h2>I have a lot of them</h2>
  <p>I do</p>
  <h2>Are you free on wednesday I want to do a sleepover</h2>
  <p> ok I will come</p>

This particular example looks… ok—but designers will cringe at this idea.

Elements in typography should not have uniform gaps between them. It’s important for readability that headings and paragraphs are less spaced apart than paragraphs and paragraphs! We’ve also just added flex to this sequence of elements that is clearly more suited to the good old fashioned block layout!

It’s not just typography either—there are quite a few situations where we might want uneven gaps between elements.

<article>, display: block, and margins

From MDN:

The <article> HTML element represents a self-contained composition in a document, page, application, or site, which is intended to be independently distributable or reusable (e.g., in syndication). Examples include: a forum post, a magazine or newspaper article, or a blog entry, a product card, a user-submitted comment, an interactive widget or gadget, or any other independent item of content.

As MDN states, an <article> is a composition of presumably different sibling elements under the <article> parent. Since the web is mostly a vertical medium, this probably means that we are using display: block (ruling out the use of gap) and desperately looking for a means to space out our elements.

Enter: CSS margins, the direct child selector, and (an absolute classic) string manipulation.

Using margins responsibly

I think that margins should be used in a way that they are in some way a property of the collection of elements. I’ll cover one way to do this when you are using tailwind and components, and one way for when you are using CSS selectors.


    <MyHeadingComponent extraClasses=”mb-1” />
    <MyParagraphComponent extraClasses=”mb-2” />
    <MyParagraphComponent extraClasses=”mb-2” />

A solution that looks a bit like the above seems to fit the bill. The margins on the individual components remain visible and obvious in the parent component.

There are many solutions to the problem of getting additional classes into an already defined component using tailwind. Since I am saying that defining margins on individual elements is a poor choice, simply defining a prop for incoming classes and concatenating them onto the end of the root element’s classes shouldn’t generate any conflicts. Of course, there is also the amazing tailwind-merge that will resolve any class conflicts if there happen to be any.

Vanilla CSS

article > h2 {
    margin-bottom: 0.5rem;

article > p {
    margin-bottom: 1rem;

Using the direct child selector we can have margins defined relative to the parent element. This achieves a similar result, we can have our margins for a composition of elements defined in one place. This might look a bit nicer using SASS.

article {
    & > h2 {
        margin-bottom: 0.5rem;
    & > p {
        margin-bottom: 1rem;

Although, when using technology like styled components, I wish I could do this:

article > MyCustomComponent {
    margin-bottom: 0.5rem;


CSS margins are a source of much debate and frustration for developers. Banishing margins altogether from our projects also doesn’t seem like a particularly functional solution. Instead, we might try our best to define margins minimally: relative to the parent of a collection of different child elements.