Character Sheet Development
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Main Article: Building Character Sheets

This is a general guide to different approaches/methods you can use to create the general layout of your custom character sheets.

Layout Types

The four main approaches/frameworks that people use for designing layouts on Roll20 character sheets.

CSS Grid

Many newer character sheet use CSS Grid for their layout, and is the recommended method doing the general layout of sections on a sheet. Aligning things in grid, columns is great, and you can create grids inside other grids.

A Complete Guide to CSS Grid
CSS Grid Garden is a great training game for learning about CSS Grid.
CSS Grid Layout MDN Web docs
sheet example on converting from table to CSS Grid^(Forum)

Example:

<div class="grid-section">
  <span>1st span</span>
  <span>2nd</span>
  <span>3rd</span>
  <span>4th, stuff</span>
  <span>5th, other</span>
  <span>6th</span>
</div>

.sheet-grid-section{
  display: grid;
  grid-template-columns: 600px 300px;
  grid-template-rows: 150px 150px 150px;
}

grid-template-areas can be used for naming sheet sections and then easily display in a human-readable way how each section is positioned in the grid. The drawback is that you can't have sections that overlap with each-other using this.

Subgrid

Subgrid is only available for Firefox, but when it's released for Chrome sheet design will get more easy as you can use the main grid lines in children, making it easier to align sub-components with main components of the sheet.

You can implement subgrid in sheets, and then create a fallback design, in case the browser doesn't support CSS Subgrid.

<div class="grid">
  <div class="item">
    <div class="subitem"></div>
  </div>
</div>

.grid {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(4, minmax(100px, auto));
}

.item {
  display: grid;
  grid-column: 2 / 7;
  grid-row: 2 / 4;
  grid-template-columns: subgrid;
  grid-template-rows: repeat(3, 80px);
}

.subitem {
  grid-column: 3 / 6;
  grid-row: 1 / 3;
}

CSS Flexbox

CSS Flexbox guide

Flexbox is a good way to align elements in rows or columns that flex and wraps around to new rows depending on the elements. Better than using the old float: right; method of aligning things. Some sheet authors use CSS Grid for bigger elements and gid-like sections of a sheet, while using Flexbox for smaller components in the sheet.

Flexbox Froggy is a great training game for learning about Flexbox.

flexbox.help

<div class="flex-section">
  <span>1st</span>
  <span>2nd</span>
  <span>3rd</span>
  <span>4th</span>
  <span>5th</span>
  <span>6th</span>
</div>

.sheet-flex-section{
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  align-items: center;
}

Roll20 columns/rows

Good for basic layouts, but if you aim for a more complex layout/design, CSS Grid and CSS Flexbox is recommended.

Roll20 provides a few basic classes you can use to organize things into a simple column-based layout. To use them, just create a div with a class of '3colrow', '2colrow', or 'row'. Then inside of that div, create a div for each column with a class of 'col'. For example, to create a 3-column layout, you would could:

<div class='3colrow'>
  <div class='col'>
    <!-- Put the content for the first column here -->
  </div>

  <div class='col'>
    <!-- Second column -->
  </div>

  <div class='col'>
    <!-- Third column -->
  </div>
</div>

HTML Table

HTML table guide – Tables - MDN Web docs

Many older sheet use HTML tables for layout, but it's harder to customize and adjust to looks of it compared to other methods, so it's generally not seen as a good idea for sheet layout.

Roll20 don't accept new sheet submissions that rely on HTML tables for design, so this option shouldn't be used if you want your sheet published. Old sheet using tables do exist in the Roll20 character sheet repository, but they where created before the rule was set in place. These older sheets shouldn't be used as templates for your own designs.

Article: Why you shouldn't use HTML tabels for layout

Pages

sheet with multiple pages/tabs

Example: Tabs

When trying to mimic the paper-version of the sheet, or the sheet starts to become too long, it's a good idea to split up content into separate tabs/pages, see Tabs.

The section above also show how you can hide areas with checkboxes, useful for temporally hiding/expanding some section for displaying more info.

Images

Main Article: Image use in character sheets

You'll likely want to use images to improve the looks of a sheet, and there are a few example of how to do so, like displaying a logo or having a nice background.