The following is to report something I already noticed in the past with regards to the max-width property. Let's see an example with a funny gif:

With 336 × 210 pixels of actual resolution I've scaled the gif at 3x its per axis resolution: 1008 × 630 pixels. Also, the max-width property is set at 100%. You will see the upper, scaled gif with wrong aspect ratio in small screens (with less than 1008 pixels of available space for the forum box).

Is it possible to set a youtube video to a % of column width.
I tried a few different ways and all I seem to be able to do is set the width in pixels.
Thanks for the help,
All the best,
Jeff

A test from youtube using video:https://youtu.be/RyJUbe-foPM width:400

If you watch this turn the VOLUME DOWN first.

Gets me 90 pixels

jeff51,

Setting the width in % as you did in the Simple Post Editor does not work, also tried a max-width attribute to no avail.

Let me say I've always been using fixed resolutions for videos via the Advanced Post Editor. Any video's optimal resolution can be obtained selecting “Nerd statistics” in the drop down right click/long tap video window menu.

Just tried setting the video width in % in the Dimensions left, horizontal box, using the Advanced Post Editor. It actually works, albeit it will look garbled while editing and Preview gets broken. Here it is set at 70% width:

Well, the width porcentual parameter does not work with videos; the forum engine fails to actually detect the correct video parameters and set its height accordingly when used that way. My advice, thus, is to set an actual valid video resolution. Here it is at 854 × 480: