When a PDF is "marked" we now generate a separate DOM that represents
the structure tree from the PDF. This DOM is inserted into the <canvas>
element and allows screen readers to walk the tree and have more
information about headings, images, links, etc. To link the structure
tree DOM (which is empty) to the text layer aria-owns is used. This
required modifying the text layer creation so that marked items are
now tracked.
Note that these changes were done automatically, using `gulp lint --fix`.
With this rule, we'll thus enforce a *consistent* formatting of zero-lengths in our CSS files.
Please find additional details about the Stylelint rule at https://stylelint.io/user-guide/rules/length-zero-no-unit
