$\def\niff{\iff\kern-1.8em/\kern1.3em}$ This method, using the \cancel macro from the package cancel, seems not to render the cancel stroke in the correct colour. Does that package always render the stroke in black? At any rate, it is practically invisible on a black background, and its only effect is to obliterate some of the symbol it is meant to be cancelling.

I instead suggest \iff\kern-1.8em/\kern1.3em within a MathJax expression. That bit of MathJax is rather bulky if you want to typeset that symbol more than once, so you could define a MathJax token for it, for example by $\def\niff{\iff\kern-1.8em/\kern1.3em}$. You may then write $P\niff Q$ for $P\niff Q$ or $1\niff2$ for $1\niff2$.

(Earlier, in this question, I had based my definition of \niff on \Longleftrightarrow as that had already been mentioned in this question. But a comment by Calvin Khor to this answer to another question on a similar subject pointed out two advantages of \iff over \Longleftrightarrow: appropriate spacing and a name which suits what the symbol means. I accordingly use \iff.

Just as a check to see if the spacing looks right: the next line below uses \iff and the line below it uses my \niff defined above.

$\begin{gather} P\iff Q\\ P\niff Q \end{gather}$

$\def\niff{\iff\kern-1.75em/\kern1.25em}$ This method, using the \cancel macro from the package cancel, seems not to render the cancel stroke in the correct colour. Does that package always render the stroke in black? At any rate, it is practically invisible on a black background, and its only effect is to obliterate some of the symbol it is meant to be cancelling.

I instead suggest \iff\kern-1.75em/\kern1.25em within a MathJax expression. That bit of MathJax is rather bulky if you want to typeset that symbol more than once, so you could define a MathJax token for it, for example by $\def\niff{\iff\kern-1.75em/\kern1.25em}$. You may then write $P\niff Q$ for $P\niff Q$ or $1\niff2$ for $1\niff2$.

(Earlier, in this question, I had based my definition of \niff on \Longleftrightarrow as that had already been mentioned in this question. But a comment by Calvin Khor to this answer to another question on a similar subject pointed out two advantages of \iff over \Longleftrightarrow: appropriate spacing and a name which suits what the symbol means. I accordingly use \iff.

Just as a check to see if the spacing looks right: the next line below uses \iff and the line below it uses my \niff defined above.

$\begin{gather} P\iff Q\\ P\niff Q \end{gather}$

This method, using the \cancel macro from the package cancel, seems not to render the cancel stroke in the correct colour. Does that package always render the stroke in black? At any rate, it is practically invisible on a black background, and its only effect is to obliterate some of the symbol it is meant to be cancelling.

$\def\niff{\Longleftrightarrow\kern-1.5em/\quad}$ I instead suggest \Longleftrightarrow\kern-1.5em/\quad within a MathJax expression. That bit of MathJax is rather bulky if you want to typeset that symbol more than once, so you could define a MathJax token for it, for example by $\def\niff{\Longleftrightarrow\kern-1.5em/\quad}$ You may then write $P\niff Q$ for $P\niff Q$ or $1\niff2$ for $1\niff2$.

$\def\niff{\iff\kern-1.8em/\kern1.3em}$ This method, using the \cancel macro from the package cancel, seems not to render the cancel stroke in the correct colour. Does that package always render the stroke in black? At any rate, it is practically invisible on a black background, and its only effect is to obliterate some of the symbol it is meant to be cancelling.

I instead suggest \iff\kern-1.8em/\kern1.3em within a MathJax expression. That bit of MathJax is rather bulky if you want to typeset that symbol more than once, so you could define a MathJax token for it, for example by $\def\niff{\iff\kern-1.8em/\kern1.3em}$. You may then write $P\niff Q$ for $P\niff Q$ or $1\niff2$ for $1\niff2$.

(Earlier, in this question, I had based my definition of \niff on \Longleftrightarrow as that had already been mentioned in this question. But a comment by Calvin Khor to this answer to another question on a similar subject pointed out two advantages of \iff over \Longleftrightarrow: appropriate spacing and a name which suits what the symbol means. I accordingly use \iff.

Just as a check to see if the spacing looks right: the next line below uses \iff and the line below it uses my \niff defined above.

$\begin{gather} P\iff Q\\ P\niff Q \end{gather}$