I went through as much of the site as humanly possible to identify markup syntax errors and things RHDN submissions used for descriptions that are currently missing.

1) HTML errors (for W3 compliance - a bit of this is only looking right because the browser is correctly assuming the erroneous markup's intentions and this can't be guaranteed in the future)

documents.php, fonts.php, games.php, hacks.php, homebrew.php, translations.php, utilities.php
The a href is missing "quotes" for both link page ID and the pagination.

editprofile.php
Birthday inputs have value declared but it's missing an attribute.

forums.php
The table around the Forum Title is missing <tr> ... </tr> around its <td> ... </td>

forums.php, thread.php
The navigation hierarchy above/below the thread(s) has an href without quotes round forum.php

gamepage.php, translationpage.php
Each table cell in the data below the picture needs a preceeding <tr>.

gamepage.php, hackpage.php, homebrewpage.php, utilitypage.php
Needs a single space between the closing quote of class and the rowspan.
Search for:

n1"rowspan

to jump to them.

homebrewpage.php
The cell with the console has a </div>, but there is no <div> to close

main.php
The a href is missing "quotes" around thread.php?announce=0
Footer's first table is missing both <tr> and <td> in the first table, and the second table has no row, and doesn't close the <td>

memberlist.php
None of the links in the filtering table a href tags have "quotes" around the URLs
The avatars in the img src tags do not have "quotes" around the userpic URLs

newreply.php
The id of the table around the post preview is undefined, but missing "" after =

mood.php
There is a space without a semicolon

&nbsp

and just after that is an extra </tr>.

profile.php
It's complaining about & not being characterized as

&

in the postsbyuser links and shop.php links

profile.php, thread.php
It's complaining about & not being characterized as

&

in the userpic

register.php
There is a </form> after the Register button, but no <form> to close.

rules.php
Much of the file does not close its <tr>s or <td>s.
The browser is complaining about &tags& as being an attempt at character designations that do not exist (should the example be replaced with

&

to shut it up and show correctly for the user, and then any real use gets replaced by the value it represents anyways?

shop.php
In the gear stats

<td class="b n2 align="center" '>

should just be

<td class="b n2 align="center">

It's complaining about & not being

&

in the item and category links.

thread.php
The table around a deleted post is missing <tr> ... </tr> around its <td> ... </td>s.
The links when selecting time and in the pagination are missing quotes around the URLs in the a href.
The quick reply's avatar select field has no space between -1" and selected=

login.php, online.php, ranks.php, stats.php
Everything checks out

2) BBCode not implemented:
I'm noticing stray BBCode in the RHDN mirror descriptions. This may not be a complete list because I only noticed "list"

[list] ... [/list] [li] ... [li]

for

<ul> ... </ul> /* unordered list */ <li> ... </li> /* listed item */

3) Not working as intended:
sprites.php
The franchise boxes aren't receiving any information.

Daily Cycle (Boom.dk) and Remastered Cycle (Nikolaj)
These are both incompatible with PHP 8 and thus show a plain page. (blackhole89's version is working. For those curious, the background color changes with the time of day between orange, purple, blue.)