First of all thanks for your reply, Nico

Nicolas Lesbats wrote:

H1 {
        font-size: 2.5em;
H1 {
        color: green}

and the two properties will be applied. Your selector will modify the
first property applied to H1, but since it doesn't notify the second,
docprop2 will be applied without changes.

In this later case, applying cascade rules, if we had same properties the last will override. Well, this is not the case, but the philosophy of the problem I think would be the same in case of equal than in case of different propierties.
Yes I agree it is simpler to accept that equal selectors inherit the propierties not specified, if you know by fact how much of them are specified.
Suppose you had, for example, a new style archive you want to apply and other archive imported that is the specified by default.
Then, the normal case would be that you wanted all the elements you specify be trated "as is". So you will have to look at the first lets say "default" cascade and overwrite all the propierties you do not want to inherit specifiying them in your style. If you don't do....:

suppose the default is:
body {

and your new cascading with that is:
body {
font-size: 20px;

You did'n noticed of the first one and there is the disaster: All In Black

But if you use class as selector it is suppossed that you know that may be conflict for that element, then you well asume that there may be another let's say parent selector that influences in propierties not declared. Well.So in  a case that you have your new cascading sheet as:

body.mybody {
font-size: 20px;
color: black
}'re assuming the consecuences (If not why did you used selector instead of directly body?)

So the only thing I'm saying is modify the Cascading Order only in one thing:

6.4.1 Cascading Order:
1 Find all declarations that apply to the element and propierty in question.
spec CSS rules
- Find all declarations that apply to the element only and with exactly the same specificity.
- Override ALL propierties for that element using weight, origin and order.

Regards, igjav