No.9's solution may be miscopied from No.10's solution.
I suggest the solution should be revised from Before to After.

9. Create a 3x3 matrix with values ranging from 0 to 8 (★☆☆)

hint: reshape

Before

nz = np.nonzero([1,2,0,0,4,0])
print(nz)

After

np.arange(0,9).reshape(3,3)

Most Java projects nowerdays are not built using javac directly; rather, they use a build tool such as Maven or Gradle. Examples of how to build a Jigsaw project (with modules and possibly module patches) using those would be very useful.

When Show extra code is checked, the guide's extra code is shown first and then the exercise's extra code is appended to it.

A guide's extra code might be used in many exercises but not all of them, and showing all of it when it's not needed adds confusion to the exercise.

This guide could benefit from this feature; I'

Need to write a set of tutorials detailing the various examples, in the style of the "simple-exercise" tutorial.

In the Challenge part of the Blocks section, for the layout image displayed the second section as:

<section>
(Heading 2)
(A paragraph of text)

It was not clear to me that this should be a h1 and not an h2. When I saw the words Heading 2, I thought that meant write a heading with h2 tag. I was able to get past the challenge by changing it to h1, but maybe it should be a little mor