To block all elements that do NOT have a specific class (e.g., X) using uBlock Origin, you can use a cosmetic filter with the :not() CSS selector.

Solution

Use the following filter in uBlock Origin's dashboard under the Custom filters tab (or in the Cosmetic filters section for a specific site):

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example.com##:not(.X)

Explanation:

• example.com → Replace with the domain(s) you want to apply the filter to. Use * for all sites.
• ## → Indicates a cosmetic (element-hiding) filter.
• :not(.X) → Selects all elements except those with the class X.

Examples:

1. Block all elements except those with class X on all sites:

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   *##:not(.X)

2. Block all elements except those with class X on example.com:

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   example.com##:not(.X)

3. Block all <div> elements except those with class X:

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   *##div:not(.X)

How to Add the Filter:

1. Open uBlock Origin dashboard.
2. Go to the "Custom filters" tab.
3. Add your filter (e.g., example.com##:not(.X)).
4. Click "Apply changes".

Notes:

• This works for cosmetic filtering (hiding elements visually).
• If you want to block network requests (e.g., scripts, images) from elements without class X, you need a different approach (e.g., using :matches-css() or script injection, which is more advanced).
• Test the filter on the site to ensure it works as expected.