The Font Settings tab in the template style editor accepts negative values for font size fields. When a negative value (e.g., -999 rem) is entered and saved, the corresponding text on the frontend becomes invisible or extremely small, causing the page layout to break and content to be unreadable. The system does not validate or reject negative font size values, allowing invalid configurations to be saved to the database.

Navigate to: Administrator > Templates > Styles

Click: "Cassiopeia Extended - Default" (edit)

Go to: Font Settings Tab

In "Body Font Size (rem)" field, enter: -999

Click: Save button

Visit frontend at: http://localhost/joomla6/

Result: Body text becomes invisible/unreadable due to negative font size

Expected Result:

Form should display validation error message

Value should not be accepted

System should enforce minimum value of 0.1 rem or higher

Actual Result:

Negative value is accepted and saved

No error message displayed

Frontend displays with invisible/broken text layout

User is unaware of the problem

Affected Fields:

Body Font Size (rem)

H1 Font Size (rem)

H2 Font Size (rem)

H3 Font Size (rem)

Root Cause:
Missing input validation - no min attribute or JavaScript validation on numeric input fields for font sizes

Impact:

User Experience: HIGH - Makes site inaccessible

Functionality: HIGH - Breaks page layout

Data Integrity: MEDIUM - Invalid data stored in database