Thank you for your answers. It reassures me to see that everyone has a rather different approach, although I think my work is closer to afincato’s. For example, it can be silly, but I never use paper and pencil! Or very occasionally to explain something, but if I have a problem, I provide a “reduced test case” with basic HTML.
You’re right, InDesign is a printing tool, but it’s also very effective: I can know everything about colors, margins, block sizes, pt or px.
But I was trained in art school and the graphic designers I know too, so we only know InDesign, Illustrator and Photoshop. The Adobe suite is really the best for everything and other software seems to me like “ersatz” …
To make a mockup on Photoshop sounds very surprising and does not seem adapted.
Some of you have the freedom to define some minor things, but not me at all! I have to respect every pixels, otherwise my collaborator will notice it. Having a lot of work with graphic designers, I can now give an aesthetic advice, and as a programmer, have advices on navigation, or UX.