"...writing a block level element inside an inline element" is wrong.
<p> should not be inside <a>.

And the links:
https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Getting_started/Cascading_and_inheritance

https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity

*

More on Specificity

https://stuffandnonsense.co.uk/archives/images/specificitywars-05v2.jpg
- a good image

https://www.smashingmagazine.com/2007/07/css-specificity-things-you-should-know/


Hierarchy
a
0.0.1

p a
0.0.2

.class
0.1.0

a.class
0.1.1

p a.class
0.1.2

.class .class
0.2.0

p.class a class
0.2.2

#id
1.0.0

a#id
1.0.1

.class a#id
1.1.1

.class .class #id
1.2.0

#id #id
2.0.0