Tabular vs proportional figures
Fixed-width numerals that keep columns and counters from jiggling, versus naturally spaced ones that read better in a sentence.
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What it is
Tabular figures give every digit the same advance width, so 1 occupies as much room as 8 and a stack of numbers lines up unit under unit. Proportional figures give each digit its natural width, which spaces better inside a sentence. Most fonts default to proportional, a few default to tabular, and a good font ships both.
Use tabular anywhere numbers sit in a column or change in place: price tables, invoices, dashboards, countdown timers, live counters, sports scores, code. The wobble you see in a ticking clock is proportional figures re-measuring the line on every tick. CSS fix: 'font-variant-numeric: tabular-nums'.
Two clarifications. Tabular versus proportional is about width, and it is a different axis from lining versus oldstyle, which is about height (oldstyle figures rise and dip around the baseline like lowercase letters). You can combine them. And do not blanket-apply tabular figures to body copy: the narrow digits float in oversized slots and the text develops visible gaps, especially around 1 and the punctuation next to it.
Ask AI for it
Apply 'font-variant-numeric: tabular-nums' to every number that lives in a column or updates in place: the pricing table, the metrics on the dashboard cards, the countdown timer, and the order totals. Leave body copy on the default proportional figures. Add 'slashed-zero' to the monospaced and data contexts, and right-align numeric table cells so the decimal points stack.