Like you see in inspector, you defined only border color but not border width. Because it is 0px, it is invisible ;)

You need to change it to

class="border border-gray-800"

"border" will by default mean border-width: 1px so if you need thicker use for example

class="border-2 border-gray-800"

or if you wanna it only on one side

class="border-right border-gray-800"

More in documentation.

Answer from chojnicki on Stack Overflow
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Reddit
reddit.com › r/tailwindcss › border won't show up
r/tailwindcss on Reddit: Border won't show up
August 22, 2023 -

I'm a beginner to Tailwind, so it's likely something simple. I can see an <input type="text" class="border border-solid border-2 border-indigo-600">, but only a faint 1px border of #e5e7eb shows up. I can see that this is under the *, ::before, ::after rule at the top of the stylesheet. My config file is customized only for the content paths, and theme and extend are empty. I read quite a bit of the Tailwind documentation, but I'm having a lot of trouble wrapping my head around how basics like color intersect with border, background, and text seeing as the latter three all incorporate color.

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Bitovi
bitovi.com › blog › why-tailwind-doesnt-work-for-me
Why Tailwind Doesn't Work For Me
January 4, 2024 - In the previous example of border, Tailwind broke it’s own convention and it did it just to save 2 keystrokes. Whether we are talking about JS variable names, CSS class names, or just code comments, character length does not matter to the final product. Your build system should be stripping out comments and minifying the rest.
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GitHub
github.com › tailwindlabs › tailwindcss › discussions › 16002
Using @layer base { border-color } is not working for TailwindCSS ver 4 · tailwindlabs/tailwindcss · Discussion #16002
@RobinMalfait I think that because I am importing a legacy tailwindcss JS config file the --color-gray-200 was not defined by default when importing @import "tailwindcss";, so by doing this I could fix the issue with the default border color:
Author   tailwindlabs
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GitHub
github.com › tailwindlabs › tailwindcss › issues › 435
Browser ignoring .border · Issue #435 · tailwindlabs/tailwindcss
March 26, 2018 - I've come across an issue where the browser is ignoring the .border class: This happens in both Chrome & Firefox.
Published   Mar 26, 2018
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Reddit
reddit.com › r/tailwindcss › can't change the border color
r/tailwindcss on Reddit: Can't change the border color
June 10, 2022 -

I'm using the tab component from tailwind elements on a project i'm working on but I'm really struggling to change the bottom border color on the active tab (i.e. the tab you click to view the contents of the tab).

https://tailwind-elements.com/docs/standard/navigation/tabs/#section-related-resources

Whatever I try, the bottom border remains blue!

Does anyone have any idea where I can change it?

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Tailwind CSS
tailwindcss.com › docs › border-style
border-style - Borders - Tailwind CSS
Use utilities like divide-dashed and divide-dotted to control the border style between child elements:
Find elsewhere
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GeeksforGeeks
geeksforgeeks.org › css › border-style-not-working-on-hover-in-tailwind-css
Border Style Not Working on Hover in Tailwind CSS? - GeeksforGeeks
July 23, 2025 - </button> </body> </html> ... In Tailwind CSS, it is very important to apply the hover class with the correct structure. If we place the hover: class in the wrong place, it will not work.
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Reddit
reddit.com › r/tailwindcss › anyone else get annoyed typing `border-[1px]` and stuff all the time in tailwind?
r/tailwindcss on Reddit: Anyone else get annoyed typing `border-[1px]` and stuff all the time in Tailwind?
October 31, 2024 -

Design systems are getting better and more standardized these days, but honestly… I still find myself typing a lot of custom values in Tailwind — like w-[2px], h-[234px], border-[1px], that kind of thing.

It’s not a huge deal, but when it happens often, it breaks your flow. So I ended up making a tiny VSCode extension that helps me speed that part up a bit.

  • w-[2px]

  • h-[200px]

  • mt-[10vh]

It’s not a big deal, but when you’re doing it constantly, it adds up.

So I made a tiny VSCode extension that lets you type shorthands like:

  • w2pw-[2px]

  • h200ph-[200px]

  • mt10vhmt-[10vh]

Just something I threw together to save a few keystrokes. Might be useful if you do a lot of custom utility work.

🔗 tw-auto-bracket on VSCode Marketplace

Would love to hear if anyone else has little tricks or extensions they use for Tailwind. Always looking to improve the workflow 🙌

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GitHub
github.com › tailwindlabs › tailwindcss › discussions › 8006
border-l-1 is not working · tailwindlabs/tailwindcss · Discussion #8006
Just use: border-l for a 1px left border, as for any other odd or custom value you can use Arbitrary Values, for example: border-l-[3px] border-l-[5px] border-l-[3em] border-l-[3rem]
Author   tailwindlabs
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Reddit
reddit.com › r/tailwindcss › hi im learning tailwindcss but border and scroll-mt- doesnt work, help!
r/tailwindcss on Reddit: Hi im Learning tailwindcss but border and scroll-mt- doesnt work, help!
April 17, 2024 -

Hello as titel says, they dont work.

Im following a guide and in the guide they work. But for example if i try to make a list with a border or add scroll margin with scroll-mt-20, Nothing happens .

Example:

<section id="rockets" class="p-6 my-12 scroll-mt-20">

-- This scroll-mt-20 does nothing.

<li class="w-2/3 sm:w-5/6 flex flex-col items-center border border-solid border-slate-900 dark:border-gray-100 bg-white py-6 px-2 rounded-3xl shadow-xl dark:bg-black"></li>

-- This does not get a border.

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Tailwind CSS
tailwindcss.com › docs › border-width
border-width - Borders - Tailwind CSS
Use border or border-<number> utilities like border-2 and border-4 to set the border width for all sides of an element:
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GitHub
github.com › mui › material-ui › issues › 37371
Tailwind border styles don't apply · Issue #37371 · mui/material-ui
May 22, 2023 - Link to live example from official tailwind+mui example: https://codesandbox.io/s/react-tailwind-borders-issue-xmig8s?file=/src/App.tsx For some reason tailwind border styles don't work.
Published   May 22, 2023
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GitHub
github.com › tailwindlabs › tailwindcss › issues › 10552
Issue with border color not overriding · Issue #10552 · tailwindlabs/tailwindcss
February 10, 2023 - Something in either 3.2.5 or 3.2.6 has messed up class cascading for border colors. The other border classes seem to be ok, but there could be other classes affected as well, idk.
Published   Feb 10, 2023