Here is a list of Sheet Author Tips for when working with character sheets & submitting your updates to Roll20, taken from the forums.

Read the documentation

Give the Building Character Sheets-article a full read, as it contains lots of information on the specific quirks & pitfalls of how Roll20 character sheets works compared to default HTML/CSS/JavaScript. Even if you know how things work.

Some Highlights:

Frameworks

Beyond just using HTML/CSS directly, you could use some pre-processors that can speed up the development, instead of directly writing HMTL/CSS. Here is a few suggestion, used by some sheet authors including Roll20 themselves.

PUG & SCSS

Suggestion by Cassie

Use PUG & SCSS & JavaScript to make sheets. PUG & SCSS are the two most useful things I've ever learned for sheet development. Character sheets which use to take hundred+ hours takes now 20-40 thanks to PUG which is a Javascript based language to write HTML. You'll save yourself a lot of redundant work by just using the power of loops & variables if nothing else.

Troubleshooting becomes much easier when you need to fix just one line in a loop rather than a dozen copy/paste html snippets.

These are compiled languages so you'll need to do that. Easily done via terminal if your tech savy or https://prepros.io/ if you're not.

Handlebars

Suggestion by Primal Zed

Forum thread

Pros:

Looks like HTML (or whatever file type you're templating), with handlebars blocks added in
Inject data values anywhere directly into the HTML
Can yield benefit with little effort - sheet authors can use as much or as little as they want
Handlebars blocks have opening and close tags similar to HTML
Easy to add new templates for re-use and organization

Cons:

Involves new syntax to learn and helpers to understand
- Built-In Helpers
- For advanced use: Additional Helpers
- For advanced use: Repeat Helper
Requires Node.js installation and some command lines

Git/GitHub

Beginner's Guide to GitHub(Roll20)
Short Git Guide - A guide to using the command line version of Git to work on your computer, instead of GitHub Desktop

Install Pull to GitHub

EVERYONE should install Pull on theirs forks!! https://wei.github.io/pull/#basic-setup GiGs found this a bit ago and it is incredibly useful for keeping forks updated. So many pull request get rejected or delayed due to a failure to keep GitHub forks in sync with the master repo. This will automate that for you.

Use Branches in GitHub

Do not make changes to your GitHub master. Instead make a branch from master to do all upgrades there. Submit pull request to the Roll20 repo directly from a branch. This is a safe way to do version control and will improve the integrity of your work.

Pattern Libraries & Components

Suggestion by Cassie

A pattern library can be your best friend if you want to make multiple sheets. Component style programming will let you save styles to be reused between sheets. With my last project I put work into my Sheet Template. This saves me a good deal of time by not needing to start from scratch every time. Its a slow work in progress were I'm reinventing much of Bootstrap.

Helpful Functions

//Convert Integers to be Negative
const convertIntegerNegative = number => number > 0 ? -Math.abs(number) : number

//Convert an object with negative numbers
const convertIntegersNegatives = numbers => {
  numbers => {
    for (let [key, value] of Object.entries(numbers)) {
      numbers[key] = convertIntegerNegative(value);
    }
    return numbers
  }
}

//Pass in eventinfo.triggerName
const findRepeatingField = trigger => trigger.split('_')[1]

//Pass in eventinfo.triggerName
const getReprowid = trigger => {
  const split = trigger.split('_');
  return `${split[0]}_${split[1]}_${split[2]}`
}

//Pass in an object keep that has the repeating section
//Example repeating_weapon_-m1czg68yzicwhfdpyys_name
const getReprowAttribute = key => {
  const getReprowid = processingFunctions.getReprowid(key)
  return key.split(`${getReprowid}_`)[1]
}

//Provide the function with an array of keys to find transations for 
//Example ['strenght', 'agility', 'willpower']
const getTranslations = translationKeys => {
  let translations = {}
  translationKeys.forEach(key => translations[`${key}`] = getTranslationByKey(key))
  return translations
}

const parseInteger = string => parseInt(string) || 0

//Use for convernting the result of getAttrs from strings into integers
const parseIntegers = numbers => {
  for (let [key, value] of Object.entries(numbers)) {
      numbers[key] = parseInt(value) || 0
  }
  return numbers  
}

const setAttributes = (update, silent) => silent && typeof update === 'object' ? setAttrs(update, {silent:true}) : typeof update === 'object' ? setAttrs(update) : console.error(`${update} is not an object`)

//returns strength from @{strenght}
const sliceAttr = attribute => attribute.slice(2, -1)

const sumIntegers = numbers => numbers.reduce((a,b) => a + b, 0)