Which CSS selector has the highest specificity?  *

In CSS, specificity determines which CSS rule is applied by the browsers. Specificity is usually the reason why your CSS-rules don't apply to some elements, even though you think they should.

The CSS selector with the highest specificity is the inline style. It is applied directly to the element to be styled using the style attribute. For example:

<p style="color: blue;">This is a paragraph.</p>

Next in the hierarchy are ID selectors. They have a higher specificity than attribute selectors, class selectors, pseudo-classes, and element types. For example:

#unique-element { color: blue; }

Then come class selectors, attribute selectors, and pseudo-classes. For example:

.my-class { color: blue; }
[data-my-attribute] { color: blue; }
:hover { color: blue; }

Finally, the selectors with the lowest specificity are element type selectors and pseudo-elements. For example:

p { color: blue; }
::before { color: blue; }

So, to summarize, the order of specificity from highest to lowest is:

1. Inline styles
2. IDs
3. Classes, attributes, and pseudo-classes
4. Elements and pseudo-elements

This is a simplified explanation. The actual calculation of specificity can get quite complex, especially when selectors are combined.