Why does .foo a:link, .foo a:visited {} selector override a:hover, a:active {} selector in CSS?

When you first started with CSS, you might have learned about the LoVe-HAte mnemonic for the order in which to specify link selectors (a:link, a:visited, a:hover, a:active). Have you ever wondered why this mnemonic was chosen?

Well, there's a note in the spec on how the link and dynamic pseudo-classes are treated when multiple rules using all of them apply to the same element, which explains why you need to set link selectors in that order:

Note that the A:hover must be placed after the A:link and A:visited rules, since otherwise the cascading rules will hide the 'color' property of the A:hover rule. Similarly, because A:active is placed after A:hover, the active color (lime) will apply when the user both activates and hovers over the A element.

Anyway, the point I'm trying to make above is that all four pseudo-classes, being pseudo-classes, have equal specificity. Everything else about specificity applies. In this case, out of a bunch of equally specific selectors, the last rule is applied. When or how each pseudo-class is triggered is never relevant.

Now, the simple introduction of the .foo selector causes your second set of link/visited rules to override your first set of link/visited styles and the hover/active styles, forcing links in elements with that class to always appear green until you add hover/active styles with the .foo selector.

^{Sorry if my answer seems stitched-up or slipshod by the way, I'm typing this on my iPhone right now and it's pretty hard to think out here...}

This is how I understand it. All these pseudo classes have same specificity, so the pseudo class written at last wins. Now what do pseudo-classes :link, :visited, :focus, :hover, :active do? Let us see one by one.

a: link{color: red} tells the user agent to color the anchor element red in any state. Run the following script:

  a:link {
  color: red;
  
  }

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The anchor element is colored red in the following states if and only if the link is unvisited,

• Unvisited
• hovered
• Focused(Tabbed)
• Active(Clicked)

So, a: link{color: red} tells the user agent to give red color to anchor element in all of the above states. Now let's compare it with a:hover pseudo-class. Run the following script

a:hover {
  color: red;
  
}

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The anchor element is colored red in the following states,

• hovered
• Active(clicked)

We see that both :link and :hover pseudo classes are capable to define the hover state -- So if you assign these two pseudo-classes to a particular element then the one mentioned at last in the css file wins. This is the reason we say :link will override :hover when the former is mentioned below the later. The same concept holds for other pseudo-classes. I'd like to give list of what every pseudo class do.

a:link {...} sets the following states of an unvisited link
- Focused(Tabbed)
- hovered
- Active(Clicked)

link state will override every other state.

a:visited {...} sets the following states of a visited link:
- Focused(Tabbed)
- hovered
- Active(Clicked)

a:visited {...} will override every other state except :link if and only if the link has been visited.

Note that visited means it must be considered visited as per user agent's cache. E.g. a website visited 10 days ago might not be in user agent's cache then it'd be technically considered unvisited.

a:focus {...} sets the following states for both visited and unvisited links:
- Focused(Tabbed)
- hovered
- Active(Clicked)

a:focus {...} overrides :hover and :active states.

a:hover {...} sets the following states to both visited and unvisited links:
- hovered
- Active(Clicked)

a:hover {...} overrides :active state

a:active {...} sets the following states for both visited and unvisited links:

• Active(Clicked)