I think that I may be much older than you, and I have fought this Balrog for decades (and if you get the reference, good for you!), so forgive me if I seem a bit intense about this. Why even bother “going in” to “test” “interfaces”, or any of the rest of it? Why go there at all? If the browser wouldn’t let you look at porn, then you wouldn’t see it anyway. If it would let you look at porn, you shouldn’t be looking at it anyway. Why do you have to KNOW? Just leave it all alone, man! Imitate King David (a man who knew a great deal about God’s attitude toward sexual sin), who in Psalm 100 sang “I did not set before my eyes any unjust thing” (DR). Or let’s use the book of Proverbs for what it’s for: "To give…to the young man knowledge and understanding (1:4): Proverbs 3: 5-8; 4:14-15; 6:27-28; 22:3. Look them up, and pray about them.
Surely you know the conditions for mortal sin: 1) grave matter (which porn is); 2) sufficient reflection (you thought about what you were going to do); 3) full consent. Here’s where it gets tricky. I am willing to believe that you were not licking your lips with anticipation at a night of lustful enjoyment. However, you have admitted that you squinted your eyes, gritted your teeth, and jumped in. At the very least you knowingly and willingly placed yourself in a near occasion of sin.
I’m not qualified to rule on your sin. Confess it and ask for advice. Then follow it without question or hesitation. Again, I beg you - just drop all this and walk away. God told Adam and Eve not to eat the apple. Would it have been mortal or venial, in your opinion, for Eve to have said, “Well, I’m not sure I’ll die - I’m not sure if it’s one of those apples - tell you what. I’ll just take a tiny bite - I won’t even taste the juice - and I’ll spit it right out”?