Update content
Decrease title size of Prolongements
Decrease font weight for words
Add download link for offline material
Revert Australia map to 5e043a2a6af7440d1ef781d934

Retrieve transparency
Delete unused file
Add alpha gradient to upper postcard layers bottom
SEO meta description
Add favicons
Specify postcard sizes in HTML
Inline scrolling to show postcard immediately
Lazy-load all diaporama images

Prioritize postcard images
Apply postcard positioning earlier through CSS
Add picture_tag plugin to decrease postcard size

Make postcard images responsive
Improve degradability of picture_tag

See https://github.com/robwierzbowski/jekyll-picture-tag/pull/50
Correct media sources precedence
Center postcard
Apply parallax effect on window resize
Allow progressive display of postcards
Do not apply parallax scrolling on small screens