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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Tailwind CSS
v2.tailwindcss.com › docs › border-opacity
Border Opacity - Tailwind CSS
You can control which variants are generated for the border opacity utilities by modifying the borderOpacity property in the variants section of your tailwind.config.js file.
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GitHub
github.com › tailwindlabs › tailwindcss › issues › 2939
Border-Transparent not aplying · Issue #2939 · tailwindlabs/tailwindcss
November 29, 2020 - i copy the default hero section from tailwind UI, and has border-transparent but when a see the live page it has a border white, the only way to solve is to change to border-none, but i want to kno...
Published   Nov 29, 2020
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Tailwind CSS
tailwind.build › classes › border-color › border-transparent
.border-transparent - Tailwind CSS class
← Tailwind CSS classes list · <div class="border-transparent border-4 p-8"></div> Click one of the examples listed below to open the Shuffle Visual Editor with the UI library that uses the selected component. Open in Visual Editor → · .border-transparent { border-color: transparent; } .border-current ·
Find elsewhere
🌐
Tailwind CSS
night-tailwindcss.vercel.app › docs › border-color
Border Color - Tailwind CSS
You can customize your color palette by editing the theme.colors section of your tailwind.config.js file, or customize just your border colors using the theme.borderColor section.
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Shuffle
shuffle.dev › tailwind › classes › border-color › border-transparent
Tailwind CSS class: border-transparent
« All Tailwind CSS classes · <div class="border-transparent border-4 p-8"></div> .border-transparent { border-color: transparent; } Click one of the examples listed below to open the Shuffle Visual Editor with the UI library that uses the selected component.
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Tailscan
tailscan.com › tailwind › borders › border-l-transparent
border-l-transparent - Tailwind Border Color - Tailscan for Tailwind CSS
border-l-transparent is a Tailwind CSS class. Check out the css, examples how to use this class and other classes for styling the Border Color in our Tailscan Library.
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GitHub
github.com › tailwindlabs › tailwindcss › issues › 8430
Border opacity does not apply to the default border color · Issue #8430 · tailwindlabs/tailwindcss
May 25, 2022 - Adding border-opacity- classes has no effect on an element when it is using the default border color. The opacity still works with other border colors. For example, there is a visible border here when I would expect it to be transparent.
Published   May 25, 2022
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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:
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GeeksforGeeks
geeksforgeeks.org › css › tailwind-css-border-opacity
Tailwind CSS Border Opacity - GeeksforGeeks
July 23, 2025 - It is the alternative to the CSS Opacity / Transparency. ... <!DOCTYPE html> <html> <head> <link href="https://unpkg.com/[email protected]/dist/tailwind.min.css" rel="stylesheet"> </head> <body class="text-center mx-4 space-y-2"> <h1 class="text-green-600 text-5xl font-bold"> GeeksforGeeks </h1> <b>Tailwind CSS Border Opacity Class</b> <div class="mx-14 bg-green-200 grid grid-rows-4 grid-flow-col gap-2 text-justify p-4"> <p class="border-green-800 border-4 border-opacity-100 p-2"> A Computer Science Portal for Geeks </p> <p class="border-green-800 border-4 border-opacity-75 p-2"> A Computer Sc
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Shuffle
shuffle.dev › tailwind › classes › background-color › bg-transparent
Tailwind CSS class: bg-transparent
« All Tailwind CSS classes · <div class="bg-transparent h-5"></div> .bg-transparent { background-color: transparent; } Click one of the examples listed below to open the Shuffle Visual Editor with the UI library that uses the selected component. Open in Visual Editor → ·
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GitHub
github.com › tailwindlabs › tailwindcss › issues › 14240
Border opacity not working when color isn't specified · Issue #14240 · tailwindlabs/tailwindcss
August 23, 2024 - https://play.tailwindcss.com/2kFJgAhjXC · Describe your issue · You need to add a border color for border-opacity-<number> to work. Just border border-opacity-<number> should work on it's own.
Published   Aug 23, 2024
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Tailscan
tailscan.com › tailwind › borders › border-t-transparent
border-t-transparent - Tailwind Border Color - Tailscan for Tailwind CSS
border-t-transparent is a Tailwind CSS class. Check out the css, examples how to use this class and other classes for styling the Border Color in our Tailscan Library.