Openning tag <h1> closing with </h3>

Couldn’t find any more appropriate place to report this issue, so hopefully it’s the right place.

I was just looking at HTML-source o view-source:https://github.com/backdrop/backdrop-issues/issues?q=is%3Aissue+is%3Aopen and noticed that one of <h1> tag is incorrectly closing with </h3>.

Here is the code:

<header class="d-sm-none SelectMenu-header pb-0 border-bottom-0 px-2 px-sm-3">
                <h1 class="f3 SelectMenu-title d-inline-flex">
                  <button
                    class="color-bg-primary border-0 px-2 py-0 m-0 Link--secondary f5"
                    aria-label="Return to menu"
                    type="button"
                    data-action="click:notifications-list-subscription-form#closeCustomDialog"
                  >
                    <svg style="" class="octicon octicon-arrow-left" height="16" viewBox="0 0 16 16" version="1.1" width="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.78 12.53a.75.75 0 01-1.06 0L2.47 8.28a.75.75 0 010-1.06l4.25-4.25a.75.75 0 011.06 1.06L4.81 7h7.44a.75.75 0 010 1.5H4.81l2.97 2.97a.75.75 0 010 1.06z"></path></svg>
                  </button>
                  Custom
                </h3>
              </header>

Here is the screenshot: