Many portable versions of office suites allow loading custom fonts from a folder.
Avoid "free fonts download" websites full of pop-ups. The safest sources are: jf banumathi font portable
If you design a local HTML page:
@font-face
font-family: 'JF Banumathi';
src: url('D:/PortableFonts/JF Banumathi.ttf');
Open the HTML file with Portable Firefox from the same drive. The font works portably. Many portable versions of office suites allow loading
Because JF Banumathi is a respected typeface, always download it from the official Junction Font repository or authorized Tamil font archives (e.g., Tamil Fonts by Junction). Look for the .ttf file. There is no separate “portable installer”—instead, you create portability yourself: Open the HTML file with Portable Firefox from the same drive