Having played some more normal difficulty puzzles on that site, here are a few things I've noticed:
- The puzzles are algorithmically generated.
- The difficulty between two puzzles in the same category can vary significantly
- Most of the "normal" 10x10 puzzles I saw (and even most "hard" 10x10 ones) are set up such that you can easily deduce where a star is within a minute of starting.
I don't know how this website generates the puzzles - they could either have slightly different methods of generating the puzzles for normal and hard, or have them pre-generated and then categorized into normal and hard. Either way, this puzzle does not feel similar in difficulty to the other normal 10x10 puzzles, so this just happens to be a puzzle where the algorithm results in a "normal" difficulty puzzle that is actually hard.
I have found a way to make some additional progress without guessing:
Consider rows 5-7, as highlighted below. We know that the three rows must have 6 stars total, and four of those will be in the areas highlighted blue.
Now consider the green area on the left - if there are two stars in that area, A5 must be a star. However that forces D7 to be a star, and with a star in I7 or J7 there cannot be a second star in the green region. So A3 or A4 must be a star, allowing us to cross off B3. We also know that there must be a star in the yellow area (E5, F5, F6).
The next step comes from the deleted comment with a small addition
If you look at the first three columns, you can determine that the two stars in the upper-left shape must be in those three columns. Additionally notice that there must be a star in B5 or C5, and we also know there must be exactly one star in D5-7.
The next step is a bit of a guess-and-check, but has motivation for checking the particular spot.
We know that there must be at least one star in F-H1. If we could get a similar shape in the upper left, it would mean there would be one star in A-C1. We try a star at A2, so A4 must be the other star in the column and B7 must be a star as well. With a star in I7 or J7, the region with C5, D5, and D6 can't fit two stars. So A2 is not a star.
So there must be a star in A-C1, and a star in F-H1. With I1 and J1 ruled out, we now know there is a star in J2.
I haven't found a good way to go from here to the end without making a guess and checking it. If you play around with it here you can get a sense for what makes this particular puzzle difficult - if you make a wrong guess it will almost work, and the error won't show up until you've got it almost completely filled in.
Update Dec 2022:
I decided to take another look at this, and did find a decent way to get to the end from where I got stuck. The main key was that I looked at where stars must be in row pairs.
Here every contiguous blue or green marking indicates where a star must be, sometimes going across regions. Notice that in the top three rows there are four places that must contain stars and one determined star. So there must be a single star between A3 and the area I've got in yellow. It's easy to see that if A4 is a star there will be multiple other stars determined, so we should check what we get if it is not A3 that has a star (and therefore somewhere in the yellow area).
From there, it makes it easy to finish the puzzle. This is still just a guess and check, but with more reason as to why I made the particular guess.
As I've done more puzzles on that site I've found quite a few where I couldn't make progress without guessing and checking. It is nice to be able to see right away a square that either cannot or must be a star, but it's also helpful to see squares that, if they either were or weren't a star, would lead to a number of immediate deductions. Those squares are useful to start with if you do need to guess and check.