Allow declaration of variable values in the same source file

[yamgent]

From #553 (comment).

index.md:

<span id="color">red</span>
<font color="{{ color }}">my text</font>

However, a better syntax is needed, because outside of variables.md, <span>s are not always variables.

---

EDIT:

Per further discussions below, support such a syntax:

index.md:

<variable id="color">red</variable>

<font color="{{ color }}">my text</font>

[damithc]

However, a better syntax is needed, because outside of variables.md, <span>s are not always variables.

One option is to consider all spans/divs as local variables. Will it affect performance?

[yamgent]

One option is to consider all spans/divs as local variables. Will it affect performance?

<div> and <span> are used quite frequently (a quick search in the MarkBind's documentation reveals around 20-40 hits per page. Extreme case can be found in usingComponents.md; 1458 spans and 60 divs).

Also <span> "variables" will get rendered on the page unless we use a d-none class.

<span id="color" class="d-none">red</span>

The use of d-none deviates from way that we declare variables in variables.md (which we never use as a page), and the variables inside <include> (which are processed so they never get rendered).

[damithc]

I'm quite happy to have a separate syntax for variables. Any suggestions?

<span id="color" var>red</span>?
<var name="color">red</var>?

[yamgent]

<var name="color">red</var>?

<var> is a legal HTML5 tag for tagging mathematical variables.

How about <value>?

<value name="color">red</value>

[damithc]

value is a bit misleading though as red is the value. property?

BTW, do we allow overriding these via the include?

[acjh]

How about <variable>? We intend to strip these out, not just hide them with CSS right?

[damithc]

How about <variable>?

Sounds good.

[jamos-tay]

Just a random suggestion, how about specifying variables in the front matter?

[damithc]

Just a random suggestion, how about specifying variables in the front matter?

That may be a bit too restrictive, given that the value of a variable may span multiple lines?

[jamos-tay]

Sure, that makes sense