|As you know, the html file you were looking for was compiled of 13 letters. I included the answers given to me by those who first solved it. A few questions had a small "error margin" to make things more difficult, but still, people will always think very differently when solving problems.
1st letter = The first letter of the name of the star of the most expensive movie of 1991. // Arnold Schwarzenegger => a.
2nd letter = The second last letter of the country where a man was born who was succeeded by Karl Dönitz. // Adolf Hitler => Austria => i.
3rd and 4th letter = These two letters you get by dividing the year when the Battle of Adrianople was fought (Constantine I and Licinius in command) by the age when a person can legally drink alcohol in Colombia, dividing the answer by the number of kidneys in a normal human being, inverting the number you get and turning that number into roman numerals. // (324/18)/2 = 9 => 6 => vi.
5th letter = A person is born on 26th of September 1983 and is of certain age on the same day of the same month in 2019. Take this age, divide it by XVIII, multiply the answer by fyra, add diez and see which letter in the alphabet corresponds to the number you get. // 36/18 * 4 + 10 = 18 => r.
6th letter = This popular man's first name becomes a word expressing gratitude when the letter you are looking for is added to the name. // (t)Hank.
7th and 8th letter = To get these two letters, you must first figure out the word for a watercraft that operates under water. From this answer, remove the part referring to the military, then remove from the remaining word the letter which is the first letter of the planet which is the sixth from the Sun and now arrange the remaining letters so that the vowel comes last. // Submarine => sub => ub => bu.
9th letter = The first letter of the name of the lead guitarist who with his band in the late 1980s released an album called Second Heat. Not the person with the initials B.B. // Paul Gilbert => p.
10th letter = The last letter of the word used to describe the opposite of low temperature. // High temperature => hot => t. (interestingly, dominikh and eZe suggested "heat", which, albeit not being the "right" answer, luckily also ends in a t.)
11th letter = Think of another word for a canine, reverse that word and choose the first letter from the popular name of an adversary related to the reversed word. // Dog => god => Satan.
12th letter = The letter which does not belong: b, n, m, k, l, c, u, q, t. // u is the only vowel.
13th letter = On a basic keyboard with a QWERTY-layout, do the following: put your finger on the CTRL on the bottom left, move up three keys, navigate three keys to the right, then up into a gap between two keys, then move 7 gaps to the right, down once over a key, four keys to the left, then choose the letter which appears first in the alphabet from the surrounding non-numerical keys, move down to a gap again from there, then move two gaps to the right and finally up over to the letter you need to complete the puzzle. // j.