Areas covered by Flexbox which are difficult or impossible to achieve with Grid

Advantage Flexbox

Here are 13 areas where flexbox comes out ahead of Grid (Level 1):

1. Centering wrapped items. Imagine five elements. Only four per row. The fifth one wraps. In a flex container, that fifth one can be easily aligned across the entire row with justify-content. Try centering this fifth item in a grid container. Not a simple matter.

   • Aligning grid items across the entire row/column (like flex items can)

   • How to center elements on the last row in CSS Grid?

   • How to offset a grid item, also shifting its siblings?

   • Can I have a varying number of columns per row in a CSS grid?

2. Wrapping. Flex items of variable lengths have no problem wrapping. Try getting grid items of variable lengths to wrap. Not possible.

   • How to get grid items of different lengths to wrap?

3. Auto margins. Flex items can be placed, packed and spaced away throughout their container with auto margins. Grid items, however, are confined to their tracks, greatly diminishing the utility of auto margins.

   • Can auto margins work in CSS Grid like they do in Flexbox?

4. Min, Max, Default – all in one. Setting the min-width, max-width and default width of a flex item is easy. How can all three lengths be set on a grid column or row? They can't.

   • Setting the minimum, maximum and default length of a grid column / row
   • Set minimum and maximum widths to grid column using percentages (related, but not exactly the same problem)

5. Sticky footer / header. It's just so much simpler and easier to pin a footer or header with flexbox.

   • How can I have a sticky footer with my CSS Grid layout?

6. Consuming remaining space. A flex item can consume remaining space with flex-grow. Grid items have no such function.

   • Aligning grid items across the entire row/column (like flex items can)

   • Make grid item use remaining space like flex item with flex-grow: 1

   • How to make the items in the last row consume remaining space in CSS Grid?

   • How to make CSS Grid last row to take up remaining space

   • Hiding a left column in CSS Grid

   • How to get the effect of grid layout's grid-template-columns with a variable number of columns?

   • CSS fr / fractional units minimum too large

7. Shrinking. Flex has flex-shrink. Grid has... nothing.

   • Shrink grid items just like flex items in css

8. Limiting the column count in a dynamic layout. With flexbox, creating a wrapping two-column grid that remains fixed at two-columns across screen sizes is no problem. In Grid, despite having all these great functions, such repeat(), auto-fill and minmax(), it can't be done.

   • Make CSS Grid auto-fill only 2 columns

   • CSS grid maximum number of columns without media queries

9. Creating space between first and last items. In a grid container with a variable number of columns, it's not easy to add an empty first and last column. Margins, padding, columns and pseudo elements each have their limitations. It's simple and easy with flexbox.

   • Add space before and after first and last grid items

10. An important benefit of the inline-level container is lost in some cases. If you have a Grid layout with a dynamic number of columns – meaning you cannot set the number of columns or a width for the container – then display: inline-grid doesn't work. All items stack in a single column. This is because the default setting on grid-auto-columns is one column. In at least some cases, flexbox fixes the problem.

    • How to make a grid container shrink to fit the content?

11. Getting columns with author-defined grid areas to wrap without media queries. Let's say you have a two-column grid containing grid areas that have set locations, and want the grid to automatically transition to a single column (with the second column wrapping below the first) on smaller screens. With grid, you would need a media query. The auto-fill and auto-fit functions will not work because the locations of grid areas have been specified. If you want to avoid a media query, flexbox's flex-wrap function may be useful.

    • Two-Column grid should wrap into One-Column grid

12. There is no column-reverse function in CSS Grid. Getting items to populate a container starting from the bottom isn't possible with a single rule applied to the grid container. With flexbox, however, the task is simple with flex-direction: column-reverse.

    • Filling cells starting from the bottom in CSS Grid

    • Does CSS Grid have a row direction similar to flexbox?

13. The resize property on a grid item has no effect on the track. Unless a column or row track is set to auto (content-based sizing), resizing a grid item will overflow the track. Since flexbox doesn't have column and row tracks, it may be a useful alternative.

    • Resize property on grid items results in overlap of other grid items

Flexbox and CSS grid are two different features and I don't agree about saying that one can replace another or that CSS grid is a superset of flexbox.

You said:

I have not found a clear explanation why Grid could not be used for 1-D and replace Flexbox.

Here is a basic flexbox example that involve wrapping that you cannot achieve (or probably difficult to achieve) using CSS grid. Reduce the window size and see how the elements will behave.

.box {
  display: flex;
  flex-wrap: wrap;
}

.box>span {
  border: 1px solid;
  padding: 10px;
  flex-grow: 1;
}

<div class="box">
  <span>some text very long here some text very long here </span>
  <span>text here</span>
  <span>A</span>
  <span>B</span>
</div>

This is a 1-D layout where each line may have elements resized differently depending on the free space without being inside a 2-D grid. It will be difficult to achieve the same output using CSS-grid.

Basically flexbox is more suited when it comes to multiline/multirow content following one direction whereas CSS grid is more about a Grid with row and columns. Of course, when it comes to only one line a 2D grid can be considerd as 1D thus flexbox and CSS grid may achieve the same thing.

It's like comparing a table with only one tr and multiple td with a set of inline-block element inside one line BUT when it comes to wrapping and multiple tr it's clear that table and inline-block are different.

Worth to note that you can achieve what you want using any techniques in general. In the past, Flexbox wasn't there and developers were able to build layout using float (Boostrap is the best example). Then flexbox come with more powerful features to make things easier. Same thing for CSS grid and for future features.

Here is an example : https://drafts.csswg.org/css-align-3/

This Draft is talking about a future enhancement of alignment where we can consider jutify-content, align-items, etc without even using flexbox or CSS grid but directly on block elements.

If implemented, will this make Flexbox and CSS Grid useless? I don't think so.