Level 1A edit
Text
Level 2A edit
Text.
Level 3A edit
Text
Level 4A edit
Text
Level 5A edit
Text
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Level 1B edit
Text
Level 2B edit
Text
Level 3B edit
Text
Level 4B edit
Text
Level 5B edit
Text
Tested with Safari on an iPhone 8 with iOS 14.4. In the desktop version (not in the mobile version or the Wikipedia app), "Level 2A" has much smaller text size than "Level 2B", and a little smaller than "Text". It appears to happen when the horizontal ruler below a level 2 heading is shortened due to content to the right, e.g. an image or infobox. Level 1, 3, 4 and 5 headings are the same size in the A and B version. All "Text" are the same size. In Firefox on Windows 10, "Level 2A" and "Level 2B" are the same expected size. This issue was reported at Wikipedia:Help desk#Viewing Wikipedia with Safari on iPhones (permanent link).
The small "Level 2A" happens above with a 220px thumb but not below in "Level 2C" with 219px.
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Level 1C edit
Text
Level 2C edit
Text
Level 3C edit
Text
Level 4C edit
Text
Level 5C edit
Text