It started as pure fixes but landed as medium changes.

Icons in the headings

• adjusted the padding of the headings (h3) to leave place for the icons
• remove the margin from the headings and put a right padding to the containing div instead (same 10px as left padding)
• remove all div.section-icon and put the icon to the headings itself, removed also the CSS-ruleset for div.section-icon

That makes the HTML-structure a bit smaller and simplifies the CSS-ruleset a bit

Screenshot thumbnail on the home page

• replace the overly nested structure with divs with a figure-figcaption combination (HTML5 rules)
• removed the div for the a and the img in it and give its task directly to the a
• removed the height from #screenshot-image because the fixed height in combination with the box-model: border-box; caused the broken layout, mentioned in Fix: correct the width of main and footer #130
• complement the width- and height-attributes of the image with CSS-rules that makes the image large as possible in the main container #index-right
• removed the placeholder for the alt-attribute of the image because the planned content is the same as the description below the image (now stored in figcaption); the text would be displayed twice when the image was broken and read twice for screenreader users (once from the alt-attribute and a second time from the description below the image). The empty alt-attribute itself has to stay there because if it would be removed, a screenreader would read the filename of the image.