The Dropdown That Was Transparent Because of One Number

The Explore dropdown in my navbar had started rendering transparent: the menu items floated over whatever was behind them, unreadable over the hero. Nothing in the console, no build warnings.

The class was bg-[#0a0a0a]/98. That reads as "near-black at 98% opacity," and Tailwind's arbitrary-value syntax suggests it should work. But the /98 opacity modifier only works for values in the configured opacity scale, which goes 0, 5, 10 ... 95, 100. 98 isn't on it, so Tailwind generated no CSS for that class at all. With no background rule, the panel was transparent, and the backdrop blur barely showed.

The fix was writing the color out: bg-[rgba(10,10,10,0.98)]. Arbitrary values in brackets always compile. The same class was on the mobile menu drawer, so that got the fix too.

This is the same kind of failure as a CSS typo: no error, just a missing rule. Utility classes feel type-checked because they autocomplete, but a class Tailwind doesn't recognize is just a string that matches nothing. Now if a style looks wrong, I check whether the rule exists in the generated CSS before I debug anything else.