Still distraught by what you mean...
What "also that is why you pointed"? How "make the code valid"? I thought you said is nothing about "accessible", but "valid". Now, suddenly, "valid" means "accessible"? Aside from your inclination towards using frameworks, your points are all... blurry.
You forget that you could make a less accessible drop down menu by choosing not to nest the second <ul> in a <li> from the first <ul>. You could just make a separate second <ul>, as a sibling element to the first <ul>, have a valid code all along, and use a slightly changed CSS to make the drop down. Like in the blue highlighted code I provided. You missed this point.
Using the second <ul> in a <li> means a DRILL DOWN menu. You missed this point also. You only provided an invalid code, where you put the second <ul> as a direct descendant for the first <ul>, which is a stupid and wrong thing to do, and than start mumbling about YUI. Did you even look at the requirements for a YUI drill down menu? Only if you use their framework extensively will a YUI menu ever become something to begin considering to use.
Make up your mind, please! And acknowledge that I provided VALID CODE ALL ALONG!