Is there a way to use < and > as delimiters to optional command or environment parameters?

Question

Say I want to define an environment like "color description" which would be the same as "description" except that one could add an optional color background to the label: \begin{colordescription}[red] ...

My question is inspired by the following: description (at least with enumitem) also has "normal" optional arguments (e.g. [style=...]) so, to ensure the new "colordescription" environment works the same way, I'd need to place the new optional arguments before (or after) the known ones, like \begin{colordescription}[red][style=...].

I'd like to visually separate the "new" optionals, which is why I thought of syntax like \begin{colordescription}<red>[style=...]: I think there are few occasions where the body of an environment would start with <, so the angle brackets wouldn't be confused with regular text. Maybe there's a way to consume the character after \begin{...} and do a special parse if this character is a <?

Answer 1

xparse allows (almost) arbitrary delimiters to commands using the d/D argument type. You can define the environment with:

\NewDocumentEnvironment{colordescription}{ D<>{white} O{} }
  {\begin{description}[format=\colorbox{#1},#2]}
  {\end{description}}

then the argument #1 will be optional, delimited by <...>, default white, and the argument #2 will be optional, delimited by [...], default empty.

\documentclass{article}
\usepackage{enumitem}
\usepackage{xcolor}
\usepackage{xparse}
\NewDocumentEnvironment{colordescription}{ D<>{white} O{} }
  {\begin{description}[format=\colorbox{#1},#2]}
  {\end{description}}
\begin{document}

\begin{colordescription}
  \item[foo] bar
\end{colordescription}

\begin{colordescription}[style=nextline]
  \item[foo] bar
\end{colordescription}

\begin{colordescription}<red>
  \item[foo] bar
\end{colordescription}

\begin{colordescription}<red>[style=nextline]
  \item[foo] bar
\end{colordescription}

\end{document}

Answer 2

EDITED to hand all 3 arguments directly to final macro, so no expansion is necessary on OP's part.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\makeatletter
\newenvironment{z}[1]{\@ifnextchar<{\zz{#1}}{\zz{#1}<Opt2 default>}}{}
\makeatother
\def\zz#1<#2>{%
  \def\argi{#1}
  \def\argii{#2}
  \zzz
}
\newcommand\zzz[1][Opt 3 default]{
  \expandafter\expandafter\expandafter\zzzz\expandafter\expandafter
  \expandafter{\expandafter\argi\expandafter}\expandafter{\argii}{#1}
}
\newcommand\zzzz[3]{
  First arg is \detokenize{#1}\\
  Bracket option is \detokenize{#2}\\
  Option argument is \detokenize{#3}  
}
\begin{document}
\begin{z}{ABC} \end{z}

\begin{z}{ABC}[\def] \end{z}

\begin{z}{ABC}<hi \mom> \end{z}

\begin{z}{ABC}<hi \mom>[\def] \end{z}
\end{document}