Context
Arabic was often viewed in the past as a single language with several dialects, but only one "correct" Standard written form: Modern Standard Arabic. Today, most scholars consider that Arabic is a "macrolanguage" with different varieties. So in 2009, several language codes were created for the main geographical varieties of Arabic. There are 31 language codes in the Arabic macrolanguage in ISO 639-3. In particular (and this is the focus of this ticket): ajp for South Levantine Arabic (Palestinian/Jordanian) and apc for North Levantine Arabic (Lebanese/Syrian). Codes for other Arabic varieties used below are: ary (Moroccan), arq (Algerian), arz (Egyptian), arb (Modern Standard Arabic), ar (de facto for Modern Standard Arabic), ara ("Arabic macrolanguage"). fa is Farsi, a non-Arabic language written with the Arabic script. (There are also 6 languages in the Judeo-Arabic macrolanguage)

List of steps to reproduce (step by step, including full links if applicable):

• On Windows 10 (didn't try other Windows versions, on the other hand, recent versions of macOS and Android look okay), click here

What happens?:

• The table displays the exact same text but tagged with different language codes, using the templates {{lang}} and {{wikt-lang}}
• On Windows it looks like this on Edge/Chrome/Chromium and like this on Firefox.
• On Windows + Chromium or Firefox, the text on the "ajp" line is narrower and harder to read (or even not readable at all), compared to the other lines. The apc line is fine on Windows + Firefox, but narrow & difficult to read on Windows + Chromium.
• On Windows, the wider renderings are Segoe UI, the narrower ones are Arial.

What should have happened instead?:

• All lines should look the same (with the exception of en, it's okay if it doesn't look good), with a broad text and a readable font.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:

• Windows 10 + latest versions of Chromium and Firefox. Screenshots:
  • Windows 10 + Chromium (ajp & apc have issues)
  • Windows 10 + Firefox  (ajp has issues)
  •  macOS (Catalina) + Chrome 91 (looks fine)
  •  Android 10 + Chrome 91 (looks fine)
  • macOS (Catalina) + Firefox 89 (looks fine)
  • macOS (Catalina) + Safari 14.1 (looks fine)

Discussion and potential solutions:
The problem was discussed on the Village pump of the English Wikipedia, then I opened ticket T286290 and I was advised by @Amire80 to open a new ticket because there were two separate issues.

Different contributors recommended different things and that's why I opened

• A pull request on GitHub: https://github.com/wikimedia/language-data/pull/165
• A ticket on CLDR to create a new ajp locale: https://unicode-org.atlassian.net/browse/CLDR-14884

On the Incubator the problem seems to be solved (for other languages using Arabic script) with Common.css. Wiktionary also has special fonts for ".Arab": Common.css. So this may be a solution for Wikipedia, but it would require to update all Common.css of all Wikimedia projects.