If this is for aligning digits in tables where some fonts (with traditional typography) render them by default with variable width (e.g. Segoe UI on Windows), you should look into CSS properties for:
font-variant-numeric: tabular-nums;
(this disables the proportional-nums default value for the numeric-spacing variant supported at least by OpenType fonts, and possibly by other font formats supported by the text renderer of your web browser for your particular platform)
No JavaScript needed! It is the cleanest way to disable the variable-width glyphs in these fonts and force them to use tabular digits (this generally uses in the same glyphs in the same font, but their leading and trailing gap is increased so the 10 digits from 0 to 9 will render at the same width; however some font may avoid the visual variable interdigit spacing and will slightly widden some digits, or could add bottom serifs to the foot of digit 1.
Note that this does not disable the variable height observed with Segoe UI (such as some digits will be x-height only like lowercase letters, others will have ascenders or descenders). These traditional digit forms may be disabled with CSS too, using
font-variant-numeric: lining-nums;
(this disables the default oldstyle-nums value for the numeric-figure variant supported at least by OpenType fonts, and by possibly other font formats supported by the text renderer of your web browser for your particular platform)
You can combine both:
font-variant-numeric: tabular-nums lining-nums;
—
The snippet below demonstrates this using a single proportional font (not monospaced!) featuring shape variants for digits, such as ‘Segoe UI’ on Windows and shows the different horizontal and vertical alignments produced.
Note that this style does not prohibit digits to change width if different styles like bold or italic is applied instead of medium roman as shown below because these will use different fonts with their own distinct metrics (this is not warrantied as well with all monospace fonts).
html { font-family: 'Segoe UI'; /* proportional with digit variants */ }
table { margin: 0; padding: 0; border: 1px solid #AAA; border-collapse: collapse; }
th, td { vertical-align:top; text-align:right; }
.unset { font-variant-numeric: unset; }
.traditional { font-variant-numeric: proportional-nums oldstyle-nums; }
.lining { font-variant-numeric: proportional-nums lining-nums; }
.tabular-old { font-variant-numeric: tabular-nums oldstyle-nums; }
.tabular-new { font-variant-numeric: tabular-nums lining-nums; }
.normal { font-variant-numeric: normal; }<table>
<tr><th>unset<td><table width="100%" class="unset">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
<tr><th>traditional<td><table width="100%" class="traditional">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
<tr><th>lining<td><table width="100%" class="lining">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
<tr><th>tabular-old<td><table width="100%" class="tabular-old">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
<tr><th>tabular-new<td><table width="100%" class="tabular-new">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
<tr><th>normal<td><table width="100%" class="normal">
<tr><td>Rs12,34,56,789.00/Gal<td><i>Difference Rs86,41,97,532.11/Gal
<tr><td>Rs98,76,54,321.11/Gal<td><b>Total Rs1,11,11,11,110.11/Gal
</table>
</table>Reference: https://developer.mozilla.org/docs/Web/CSS/font-variant-numeric