Cheat Sheet & Build

• There should be some extra notes in the cheat sheet. For example, there should definitely be a note explaining when to go Iceborn Gauntlet vs Riftmaker in short. (Yes, even though it's in the guide itself not far below.) Or for the different starting items: when am I supposed to go which option? Same thing applies to the "early rush" section. For your full builds you already apply the notes in a decent fashion.
  
  
• I really recommend splitting the viable legendary items into smaller categories to bring some order. (At the very least Defensive/Offensive for example.)

Items

• The item section overall seems good. Fairly short and to the point, but that's a good thing. Everything is clearly explained.
  
  
• I am just wondering about the lack of some items: are Rabadon's Deathcap, Shadowflame and Banshee's Veil never good on Mordekaiser? I can kinda see that you wouldn't like Rabadon's Deathcap for its lack of survivabilty, but it's still THE offensive item for AP champions. And what about Shadowflame in certain heavy shielding matchups like Tahm Kench, Sett or Shen? And Banshee's Veil sounds like an overall decent item with both offensive and defensive capacities.
  
  
• I think it might be a good idea to combine the different item chapters (starting, early, legendary) into a single chapter. Would make a bit more sense imo.

Mechanics/Combos

• The Q-flash combo seems to be working somewhat inconsistently? I've been fiddling around with it in practice tool a bit, but it seems to rely on the timing? For example, you can still Q-flash forward to extend the range of Obliterate.

Laning Phase/Post-laning-phase

• Really informative chapters, especially the parts about wave management. You could be adding some images to clarify some mechanics like freezing, but other from that, good job.
  
  
• About the splitpushing: if you're playing without Teleport, isn't this strategy a lot riskier? If the enemy doesn't come for you and you're walking to Baron pit, wouldn't you get there way too late to contribute to a potential fight? The enemy team will have quite a window to force the 5v4 no? And how would you react to the enemy toplaner splitpushing to pressure Baron (while they have Teleport)?

Runes

• Overall it's okay, but I wouldn't touch upon the runes that you'll never ever use. So especially the domination rune tree. There's no real reason to talk about it in the guide at all. At most you could add a couple lines without the tier list or anything.

Matchups

• Quinn: wouldn't it be important to add that she will probably roam if you play safe and freeze?
  
  
• Tryndamere: Are you sure Mordekaiser outscales him? It's still a meelee ADC after all.
  
  
• Jayce: He doesn't seem to use Phase Rush much anymore. Most players go for First Strike or Conqueror nowadays.
  
  
• Overall it's pretty good, very informative.

Guide Structure

• The guide can be massively improved already by bringing a bit more logic to the overall structure. An overall good order is: basic pre-game setup/knowledge (pros/cons, summoner spells, runes, ability overview), in game (items, gameplan) and then possibly more advanced topics.
  
  
• The rune chapter should be somewhere at the beginning, not right at the end. It's something to decide on before the game starts, while being really crucial information to play Mordekaiser.
  
  
• You could combine the mechanics/combos and ult usage sections into a single chapter while streamlining it. Right now it contains a lot of different bits of information, the order is not bad, but could be better. You could for example organize the chapter by starting with an overview of the abilities, with specific tips and tricks (ult usage, the W trick with Last Stand), then the combos.
  
  
• A skin chapter isn't really necessary as it doesn't really add any educational value. Some authors have strong opinions against them, keep that in mind, but if you like it then keep it.

Presentation

• I feel neutral about the current presentation of the guide. Everything is easy to read and the formatting is fairly consistent, but not more than that. A bit of tables/columns and some basic visuals could already work wonders. There are several resources on the site that can help with that, there are lots of templates to use for free.
  
  
• Instead of using several colors for items and other stuff, while having no color at all for other headers, I'd recommend picking a color scheme of 2/3 colors and sticking to those to make it more consistent.
  
  
• Would be great if you could center the icons and headers above the lists in the strengths/weaknesses chapter.

Conclusion
Overall this is a fairly solid attempt for a first guide. The content is good, I learnt a thing or two about Mordekaiser while reviewing. Now it's mainly about improving what you already have, adding more content and refining some of the structure. Some eyecandy with banners/cool formatting can also really help to get more readers (speaking from experience).

I'll give you an upvote for now, the guide got potential and I hope you'll be able to improve it even further.